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/chat_completion_helpers.py

@@ -1296,7 +1296,7 @@ def handle_max_iterations(agent, messages: list, api_call_count: int) -> str:
             for internal_key in [k for k in api_msg if isinstance(k, str) and k.startswith("_")]:
                 api_msg.pop(internal_key, None)
             if _needs_sanitize:
-                agent._sanitize_tool_calls_for_strict_api(api_msg)
+                agent._sanitize_tool_calls_for_strict_api(api_msg, model=agent.model)
             api_messages.append(api_msg)
 
         effective_system = agent._cached_system_prompt or ""