fix(agent): strip <think> blocks from stored assistant content

Inline reasoning tags in an assistant message's content field leak to every downstream consumer: messaging platforms (#8878, #9568), API replay of prior turns, session transcript, CLI recap, generated session titles, and context compression.  _extract_reasoning() already captures the reasoning text into msg['reasoning'] separately, so the raw tags in content are redundant.

Stripping once at the storage boundary in _build_assistant_message() cleans the content for every downstream path in one place — no per-platform or per-path stripper needed.  Measured impact on a real MiniMax M2.7-highspeed session (per @luoyejiaoe-source, #9306): 55% of assistant messages started with <think> blocks, 51/100 session titles were polluted, 16% content-size reduction.

3 new regression tests in TestBuildAssistantMessage: closed-pair strip with reasoning capture, no-think-tag passthrough, and unterminated-block strip.

Resolves #8878 and #9568.

Originally proposed as PR #9250.

run_agent.py

@@ -7294,6 +7294,20 @@ class AIAgent:
         if reasoning_text:
             reasoning_text = _sanitize_surrogates(reasoning_text)
 
+        # Strip inline reasoning tags (<think>…</think> etc.) from the stored
+        # assistant content.  Reasoning was already captured into
+        # ``reasoning_text`` above (either from structured fields or the
+        # inline-block fallback), so the raw tags in content are redundant.
+        # Leaving them in place caused reasoning to leak to messaging
+        # platforms (#8878, #9568), inflate context on subsequent turns
+        # (#9306 observed 16% content-size reduction on a real MiniMax
+        # session), and pollute generated session titles.  One strip at the
+        # storage boundary cleans content for every downstream consumer:
+        # API replay, session transcript, gateway delivery, CLI display,
+        # compression, title generation.
+        if isinstance(_san_content, str) and _san_content:
+            _san_content = self._strip_think_blocks(_san_content).strip()
+
         msg = {
             "role": "assistant",
             "content": _san_content,