Fix native tool-calling follow-up round on Gemini and Ollama (#867)

The agent's multi-round (tool-result) follow-up request was rejected with
HTTP 400 on two providers, so tools ran but the agent never produced an answer:

- OpenAI-compatible streaming (Gemini 3) dropped the per-call thought_signature
  and collided parallel tool calls, which arrive with index=None: they all
  landed in slot 0, overwriting the first call's name and corrupting its
  arguments by concatenation, so the follow-up request 400'd. Capture and replay
  each call's extra_content (thought_signature), and give every parallel call
  its own accumulator slot (allocated above the max key, so sparse or mixed
  indices can't collide).
- Native Ollama /api/chat expects object tool-call arguments, but Odysseus
  carries them as a JSON string, which Ollama rejected ("Value looks like
  object, but can't find closing '}' symbol"). Convert them to objects in the
  Ollama payload builder.

Both compose with the no-prose null-content sanitize fix from #862.

Tested: python -m pytest tests/test_llm_core_streaming.py
tests/test_llm_core_ollama.py tests/test_agent_loop.py (53 pass), and
python -m py_compile src/llm_core.py src/agent_loop.py.

src/agent_loop.py

@@ -1123,6 +1123,11 @@ def _append_tool_results(
                     "name": tc.get("name", ""),
                     "arguments": tc.get("arguments", "{}"),
                 },
+                # Gemini 3 requires the opaque thought_signature it returned with
+                # each function call to be echoed back on the follow-up turn, or
+                # the next request 400s. Replay it when present; other providers
+                # never emit it (their payload builders just ignore the field).
+                **({"extra_content": tc["extra_content"]} if tc.get("extra_content") else {}),
             }
             for j, tc in enumerate(native_tool_calls)
         ]