fix(provider): restore reasoning_content and extra_content in message sanitization

reasoning_content and extra_content were accidentally dropped from
_ALLOWED_MSG_KEYS.

Also fix session/manager.py to include reasoning_content when building
LLM messages from session history, so the field is not lost across
turns.

Without this fix, providers such as Kimi, emit reasoning_content in
assistant messages will have it stripped on the next request, breaking
multi-turn thinking mode.

Fixes: https://github.com/HKUDS/nanobot/issues/2777
Signed-off-by: Lingao Meng <menglingao@xiaomi.com>

nanobot/providers/openai_compat_provider.py

@@ -21,6 +21,7 @@ if TYPE_CHECKING:
 
 _ALLOWED_MSG_KEYS = frozenset({
     "role", "content", "tool_calls", "tool_call_id", "name",
+    "reasoning_content", "extra_content",
 })
 _ALNUM = string.ascii_letters + string.digits