fix(transport): strip Hermes-internal scaffolding keys before chat.completions

The empty-response recovery path in run_agent.py appends synthetic
messages tagged with _empty_recovery_synthetic (and the agent loop uses
_thinking_prefill / _empty_terminal_sentinel similarly). These are
internal bookkeeping markers — they must never reach the wire.

chat_completions' convert_messages only stripped Codex Responses leak
fields (codex_reasoning_items, call_id, etc.), not these _-prefixed
markers. Permissive providers (real OpenAI, Anthropic) silently ignore
unknown message keys so the bug stayed hidden, but strict
OpenAI-compatible gateways reject them outright. Observed against
codex.nekos.me:

  502: [ObjectParam] [input[617]._empty_recovery_synthetic]
       [unknown_parameter] Unknown parameter:
       '_empty_recovery_synthetic'

Because the synthetic messages persist in the session, every
subsequent request in that session carries the poisoned key and
fails identically — a deterministic 502 the retry loop mistakes for
a transient server error.

Fix: convert_messages now drops any top-level message key starting
with '_'. OpenAI's message schema has no '_'-prefixed fields, so this
is safe and future-proofs against new internal markers.

Origin: local-author
Upstream-PR: none
Patch-State: local-only

agent/transports/chat_completions.py

@@ -113,9 +113,8 @@ class ChatCompletionsTransport(ProviderTransport):
         self, messages: list[dict[str, Any]], **kwargs
     ) -> list[dict[str, Any]]:
         """Messages are already in OpenAI format — strip internal fields
-        that strict chat-completions providers reject with HTTP 400/422.
-
-        Strips:
+        that strict chat-completions providers reject with HTTP 400/422
+        (or, in the case of some OpenAI-compatible gateways, 5xx):
 
         - Codex Responses API fields: ``codex_reasoning_items`` /
           ``codex_message_items`` on the message, ``call_id`` /
@@ -127,6 +126,16 @@ class ChatCompletionsTransport(ProviderTransport):
           ``Extra inputs are not permitted, field: 'messages[N].tool_name'``.
           Permissive providers (OpenRouter, MiniMax) silently ignore the
           field, which masked the bug for months.
+        - Hermes-internal scaffolding markers — any top-level message key
+          starting with ``_`` (e.g. ``_empty_recovery_synthetic``,
+          ``_empty_terminal_sentinel``, ``_thinking_prefill``). These are
+          bookkeeping flags the agent loop attaches to messages so the
+          persistence layer can later strip its own scaffolding; they must
+          never reach the wire. Permissive providers (real OpenAI,
+          Anthropic) silently drop unknown message keys, but strict
+          gateways (e.g. opencode-go, codex.nekos.me) reject with
+          ``Extra inputs are not permitted, field: 'messages[N]._empty_recovery_synthetic'``,
+          which then poisons every subsequent request in the session.
         """
         needs_sanitize = False
         for msg in messages:
@@ -139,6 +148,9 @@ class ChatCompletionsTransport(ProviderTransport):
             ):
                 needs_sanitize = True
                 break
+            if any(isinstance(k, str) and k.startswith("_") for k in msg):
+                needs_sanitize = True
+                break
             tool_calls = msg.get("tool_calls")
             if isinstance(tool_calls, list):
                 for tc in tool_calls:
@@ -160,6 +172,11 @@ class ChatCompletionsTransport(ProviderTransport):
             msg.pop("codex_reasoning_items", None)
             msg.pop("codex_message_items", None)
             msg.pop("tool_name", None)
+            # Drop all Hermes-internal scaffolding markers (``_``-prefixed).
+            # OpenAI's message schema has no ``_``-prefixed fields, so this
+            # is safe and future-proofs against new markers being added.
+            for key in [k for k in msg if isinstance(k, str) and k.startswith("_")]:
+                msg.pop(key, None)
             tool_calls = msg.get("tool_calls")
             if isinstance(tool_calls, list):
                 for tc in tool_calls: