fix: empty response recovery for reasoning models (mimo, qwen, GLM) (#8609)

Three fixes for the (empty) response bug affecting open reasoning models:

1. Allow retries after prefill exhaustion — models like mimo-v2-pro always
   populate reasoning fields via OpenRouter, so the old 'not _has_structured'
   guard on the retry path blocked retries for EVERY reasoning model after
   the 2 prefill attempts.  Now: 2 prefills + 3 retries = 6 total attempts
   before (empty).

2. Reset prefill/retry counters on tool-call recovery — the counters
   accumulated across the entire conversation, never resetting during
   tool-calling turns.  A model cycling empty→prefill→tools→empty burned
   both prefill attempts and the third empty got zero recovery.  Now
   counters reset when prefill succeeds with tool calls.

3. Strip think blocks before _truly_empty check — inline <think> content
   made the string non-empty, skipping both retry paths.

Reported by users on Telegram with xiaomi/mimo-v2-pro and qwen3.5 models.
Reproduced: qwen3.5-9b emits tool calls as XML in reasoning field instead
of proper function calls, causing content=None + tool_calls=None + reasoning
with embedded <tool_call> XML.  Prefill recovery works but counter
accumulation caused permanent (empty) in long sessions.

run_agent.py

@@ -9736,12 +9736,25 @@ class AIAgent:
                     
                     # Pop thinking-only prefill message(s) before appending
                     # (tool-call path — same rationale as the final-response path).
+                    _had_prefill = False
                     while (
                         messages
                         and isinstance(messages[-1], dict)
                         and messages[-1].get("_thinking_prefill")
                     ):
                         messages.pop()
+                        _had_prefill = True
+
+                    # Reset prefill counter when tool calls follow a prefill
+                    # recovery.  Without this, the counter accumulates across
+                    # the whole conversation — a model that intermittently
+                    # empties (empty → prefill → tools → empty → prefill →
+                    # tools) burns both prefill attempts and the third empty
+                    # gets zero recovery.  Resetting here treats each tool-
+                    # call success as a fresh start.
+                    if _had_prefill:
+                        self._thinking_prefill_retries = 0
+                        self._empty_content_retries = 0
 
                     messages.append(assistant_msg)
                     self._emit_interim_assistant_message(assistant_msg)
@@ -9917,16 +9930,23 @@ class AIAgent:
                             self._save_session_log(messages)
                             continue
 
-                        # ── Empty response retry (no reasoning) ──────
-                        # Model returned nothing — no content, no
-                        # structured reasoning, no tool calls.  Common
-                        # with open models (transient provider issues,
-                        # rate limits, sampling flukes).  Retry up to 3
-                        # times before attempting fallback.  Skip when
-                        # content has inline <think> tags (model chose
-                        # to reason, just no visible text).
-                        _truly_empty = not final_response.strip()
-                        if _truly_empty and not _has_structured and self._empty_content_retries < 3:
+                        # ── Empty response retry ──────────────────────
+                        # Model returned nothing usable.  Retry up to 3
+                        # times before attempting fallback.  This covers
+                        # both truly empty responses (no content, no
+                        # reasoning) AND reasoning-only responses after
+                        # prefill exhaustion — models like mimo-v2-pro
+                        # always populate reasoning fields via OpenRouter,
+                        # so the old `not _has_structured` guard blocked
+                        # retries for every reasoning model after prefill.
+                        _truly_empty = not self._strip_think_blocks(
+                            final_response
+                        ).strip()
+                        _prefill_exhausted = (
+                            _has_structured
+                            and self._thinking_prefill_retries >= 2
+                        )
+                        if _truly_empty and (not _has_structured or _prefill_exhausted) and self._empty_content_retries < 3:
                             self._empty_content_retries += 1
                             logger.warning(
                                 "Empty response (no content or reasoning) — "