fix: strip extra_content from tool_calls for strict APIs (Fireworks, Mistral)

Fireworks/Mistral reject HTTP 400 'Extra inputs are not permitted, field:
messages[N].tool_calls[M].extra_content' on any session whose history
contains prior Gemini tool calls. Gemini 3 thinking models attach
extra_content (thought_signature) to tool_calls; it survived to the wire
because the sanitize paths only stripped call_id/response_item_id.

Strip extra_content from the outgoing wire copy in both sanitize paths
(ChatCompletionsTransport.convert_messages + _sanitize_tool_calls_for_strict_api),
but gate it on the target model: keep extra_content for Gemini-family
targets (the thought_signature MUST be replayed or Gemini 400s), strip it
for everyone else — including non-Gemini models that inherit a stale Gemini
signature earlier in a mixed-provider session. Native Gemini is unaffected
(GeminiNativeClient bypasses these paths).

Original stored history is never mutated (only the per-call copy).

Fixes #17986.

agent/conversation_loop.py

@@ -982,7 +982,7 @@ def run_conversation(
             # Uses new dicts so the internal messages list retains the fields
             # for Codex Responses compatibility.
             if agent._should_sanitize_tool_calls():
-                agent._sanitize_tool_calls_for_strict_api(api_msg)
+                agent._sanitize_tool_calls_for_strict_api(api_msg, model=agent.model)
             # Keep 'reasoning_details' - OpenRouter uses this for multi-turn reasoning context
             # The signature field helps maintain reasoning continuity
             api_messages.append(api_msg)