fix(aux): honor model.default_headers on auxiliary client too (#40033)

The salvaged main-agent fix (sanidhyasin) applies model.default_headers
to the primary OpenAI client, but the auxiliary client (title generation,
context compression, vision routing) builds its own clients and did not
read the override. For a `provider: custom` endpoint behind a gateway/WAF
that rejects the OpenAI SDK's identifying headers, the main turn would
succeed while auxiliary calls to the same endpoint still failed with the
opaque 502/4xx from #40033.

Add agent.auxiliary_client._apply_user_default_headers() (user values win
over provider/SDK defaults; no-op when unconfigured) and apply it at every
OpenAI-wire client construction site:
- _try_custom_endpoint() — config-level `model.provider: custom`
- the named custom-provider branch (custom_providers/providers entries),
  including the anthropic-SDK-missing OpenAI-wire fallback
- the api-key-provider, async-conversion, and main resolve_provider_client
  fallback branches

To prevent the two clients ever drifting on precedence/value handling,
AIAgent._apply_user_default_headers (run_agent.py) now delegates the config
read + merge to this shared helper (run_agent already imports from
auxiliary_client). Native Anthropic/Bedrock branches are untouched (they
don't use the OpenAI wire).

8 new tests (helper semantics + config-level custom + named custom);
full aux + attribution header suites green (295).

agent/auxiliary_client.py

@@ -357,6 +357,35 @@ _OR_HEADERS_BASE = {
 _TRUTHY_ENV_VALUES = frozenset({"1", "true", "yes", "on"})
 
 
+def _apply_user_default_headers(headers: dict | None) -> dict | None:
+    """Merge user-configured ``model.default_headers`` onto resolved headers.
+
+    User values take precedence over provider/SDK defaults, mirroring the main
+    agent client (``AIAgent._apply_user_default_headers``). This lets a
+    ``custom`` OpenAI-compatible endpoint behind a gateway/WAF that rejects the
+    OpenAI SDK's identifying headers (``User-Agent: OpenAI/Python ...``,
+    ``X-Stainless-*``) override them for auxiliary calls too — otherwise the
+    main turn would succeed but title/compression/vision calls to the same
+    endpoint would still fail. (#40033)
+
+    Returns the merged dict, or the original ``headers`` (possibly ``None``)
+    when nothing is configured. No allocation when there are no overrides.
+    """
+    try:
+        from hermes_cli.config import cfg_get, load_config
+        user_headers = cfg_get(load_config(), "model", "default_headers")
+    except Exception:
+        return headers
+    if not isinstance(user_headers, dict) or not user_headers:
+        return headers
+    merged = dict(headers or {})
+    for key, value in user_headers.items():
+        if value is None:
+            continue
+        merged[str(key)] = str(value)
+    return merged or headers
+
+
 def build_or_headers(or_config: dict | None = None) -> dict:
     """Build OpenRouter headers, optionally including response-cache headers.
 
@@ -1495,6 +1524,9 @@ def _resolve_api_key_provider() -> Tuple[Optional[OpenAI], Optional[str]]:
                         extra["default_headers"] = dict(_ph_aux.default_headers)
                 except Exception:
                     pass
+            _merged_aux = _apply_user_default_headers(extra.get("default_headers"))
+            if _merged_aux:
+                extra["default_headers"] = _merged_aux
             _client = OpenAI(api_key=api_key, base_url=base_url, **extra)
             _client = _maybe_wrap_anthropic(_client, model, api_key, raw_base_url)
             return _client, model
@@ -1532,6 +1564,9 @@ def _resolve_api_key_provider() -> Tuple[Optional[OpenAI], Optional[str]]:
                     extra["default_headers"] = dict(_ph_aux2.default_headers)
             except Exception:
                 pass
+        _merged_aux2 = _apply_user_default_headers(extra.get("default_headers"))
+        if _merged_aux2:
+            extra["default_headers"] = _merged_aux2
         _client = OpenAI(api_key=api_key, base_url=base_url, **extra)
         _client = _maybe_wrap_anthropic(_client, model, api_key, raw_base_url)
         return _client, model
@@ -1922,6 +1957,13 @@ def _try_custom_endpoint() -> Tuple[Optional[Any], Optional[str]]:
     logger.debug("Auxiliary client: custom endpoint (%s, api_mode=%s)", model, custom_mode or "chat_completions")
     _clean_base, _dq = _extract_url_query_params(custom_base)
     _extra = {"default_query": _dq} if _dq else {}
+    # User-configured model.default_headers override the SDK's identifying
+    # headers (User-Agent: OpenAI/Python ..., X-Stainless-*) on this custom
+    # endpoint's auxiliary calls too — matching the main agent client so the
+    # whole session reaches a gateway/WAF that rejects the SDK fingerprint. (#40033)
+    _custom_headers = _apply_user_default_headers(None)
+    if _custom_headers:
+        _extra["default_headers"] = _custom_headers
     if custom_mode == "codex_responses":
         real_client = OpenAI(api_key=custom_key, base_url=_clean_base, **_extra)
         return CodexAuxiliaryClient(real_client, model), model
@@ -3291,6 +3333,9 @@ def _to_async_client(sync_client, model: str, is_vision: bool = False):
                     async_kwargs["default_headers"] = dict(_ph_async.default_headers)
         except Exception:
             pass
+    _merged_async = _apply_user_default_headers(async_kwargs.get("default_headers"))
+    if _merged_async:
+        async_kwargs["default_headers"] = _merged_async
     return AsyncOpenAI(**async_kwargs), model
 
 
@@ -3578,6 +3623,9 @@ def resolve_provider_client(
                         extra["default_headers"] = dict(_ph_custom.default_headers)
                 except Exception:
                     pass
+            _merged_custom = _apply_user_default_headers(extra.get("default_headers"))
+            if _merged_custom:
+                extra["default_headers"] = _merged_custom
             client = OpenAI(api_key=custom_key, base_url=_clean_base, **extra)
             client = _wrap_if_needed(client, final_model, custom_base, custom_key)
             return (_to_async_client(client, final_model, is_vision=is_vision) if async_mode
@@ -3654,6 +3702,9 @@ def resolve_provider_client(
                     raw_base_for_wrap = custom_base
                 _clean_base2, _dq2 = _extract_url_query_params(openai_base)
                 _extra2 = {"default_query": _dq2} if _dq2 else {}
+                _headers2 = _apply_user_default_headers(_extra2.get("default_headers"))
+                if _headers2:
+                    _extra2["default_headers"] = _headers2
                 logger.debug(
                     "resolve_provider_client: named custom provider %r (%s, api_mode=%s)",
                     provider, final_model, entry_api_mode or "chat_completions")
@@ -3676,6 +3727,9 @@ def resolve_provider_client(
                         _fallback_base = _to_openai_base_url(custom_base)
                         _fb_clean, _fb_dq = _extract_url_query_params(_fallback_base)
                         _fb_extra = {"default_query": _fb_dq} if _fb_dq else {}
+                        _fb_headers = _apply_user_default_headers(_fb_extra.get("default_headers"))
+                        if _fb_headers:
+                            _fb_extra["default_headers"] = _fb_headers
                         client = OpenAI(api_key=custom_key, base_url=_fb_clean, **_fb_extra)
                         return (_to_async_client(client, final_model, is_vision=is_vision) if async_mode
                                 else (client, final_model))
@@ -3824,6 +3878,9 @@ def resolve_provider_client(
                     headers.update(_ph_main.default_headers)
             except Exception:
                 pass
+        _merged_main = _apply_user_default_headers(headers)
+        if _merged_main:
+            headers = _merged_main
         client = OpenAI(api_key=api_key, base_url=base_url,
                         **({"default_headers": headers} if headers else {}))