refactor: tie api_mode to provider config instead of env var (#1656)

Remove HERMES_API_MODE env var. api_mode is now configured where the
endpoint is defined:

- model.api_mode in config.yaml (for the active model config)
- custom_providers[].api_mode (for named custom providers)

Replace _get_configured_api_mode() with _parse_api_mode() which just
validates a value against the whitelist without reading env vars.

Both paths (model config and named custom providers) now read api_mode
from their respective config entries rather than a global override.

hermes_cli/runtime_provider.py

@@ -33,21 +33,15 @@ def _get_model_config() -> Dict[str, Any]:
     return {}
 
 
-def _get_configured_api_mode(model_cfg: Optional[Dict[str, Any]] = None) -> Optional[str]:
-    """Return an optional API mode override from env or config.
+_VALID_API_MODES = {"chat_completions", "codex_responses"}
 
-    Allows custom OpenAI-compatible endpoints to opt into codex_responses
-    mode via HERMES_API_MODE env var or model.api_mode in config.yaml,
-    without requiring the OpenAI Codex OAuth provider path.
-    """
-    candidate = os.getenv("HERMES_API_MODE", "").strip().lower()
-    if not candidate:
-        cfg = model_cfg if isinstance(model_cfg, dict) else _get_model_config()
-        raw = cfg.get("api_mode")
-        if isinstance(raw, str):
-            candidate = raw.strip().lower()
-    if candidate in {"chat_completions", "codex_responses"}:
-        return candidate
+
+def _parse_api_mode(raw: Any) -> Optional[str]:
+    """Validate an api_mode value from config. Returns None if invalid."""
+    if isinstance(raw, str):
+        normalized = raw.strip().lower()
+        if normalized in _VALID_API_MODES:
+            return normalized
     return None
 
 
@@ -104,11 +98,15 @@ def _get_named_custom_provider(requested_provider: str) -> Optional[Dict[str, An
         menu_key = f"custom:{name_norm}"
         if requested_norm not in {name_norm, menu_key}:
             continue
-        return {
+        result = {
             "name": name.strip(),
             "base_url": base_url.strip(),
             "api_key": str(entry.get("api_key", "") or "").strip(),
         }
+        api_mode = _parse_api_mode(entry.get("api_mode"))
+        if api_mode:
+            result["api_mode"] = api_mode
+        return result
 
     return None
 
@@ -139,7 +137,7 @@ def _resolve_named_custom_runtime(
 
     return {
         "provider": "openrouter",
-        "api_mode": _get_configured_api_mode() or "chat_completions",
+        "api_mode": custom_provider.get("api_mode", "chat_completions"),
         "base_url": base_url,
         "api_key": api_key,
         "source": f"custom_provider:{custom_provider.get('name', requested_provider)}",
@@ -208,11 +206,10 @@ def _resolve_openrouter_runtime(
         )
 
     source = "explicit" if (explicit_api_key or explicit_base_url) else "env/config"
-    api_mode = _get_configured_api_mode(model_cfg) or "chat_completions"
 
     return {
         "provider": "openrouter",
-        "api_mode": api_mode,
+        "api_mode": _parse_api_mode(model_cfg.get("api_mode")) or "chat_completions",
         "base_url": base_url,
         "api_key": api_key,
         "source": source,