fix(tts): strip <think> reasoning blocks from TTS text

When /reasoning show is enabled, the model's <think> blocks appear in
the final assistant message. TTS reads these aloud, which is unwanted —
users want to see reasoning but not hear it spoken.

- Add _THINK_BLOCK regex to _strip_markdown_for_tts() (tools/tts_tool.py)
  — the general TTS text-preparation path used by all TTS providers
- Add <think> stripping to prepare_tts_text() (gateway/platforms/base.py)
  — the gateway auto-TTS path
- Reuses the existing regex pattern from stream_tts_to_speaker()

Closes #34213

gateway/platforms/base.py

@@ -3833,9 +3833,10 @@ class BasePlatformAdapter(ABC):
     def prepare_tts_text(self, text: str) -> str:
         """Prepare a spoken script for TTS.
 
-        Auto-TTS should not feed raw chat Markdown or compact symbols to the
-        speech provider.  It should receive a transcript-like script: headings
-        and bullets flattened into sentence pauses, and units like ``°C``
+        Auto-TTS should not feed raw chat Markdown, ``<think>`` reasoning
+        blocks, or compact symbols to the speech provider.  It should receive
+        a transcript-like script: reasoning blocks removed, headings and
+        bullets flattened into sentence pauses, and units like ``°C``
         expanded to words such as ``degrees Celsius``.
         """
         try:
@@ -3843,6 +3844,7 @@ class BasePlatformAdapter(ABC):
             return prepare_spoken_text(text, max_chars=4000)
         except Exception:
             # Keep auto-TTS best-effort if the normalizer ever fails.
+            text = re.sub(r'<think[\s>].*?</think>', ' ', text, flags=re.DOTALL)
             return re.sub(r'[*_`#\[\]()]', '', text)[:4000].strip()
 
     async def play_tts(

tools/tts_tool.py

@@ -3057,9 +3057,14 @@ _EMOJI = re.compile(
     '[\U0001F000-\U0001FAFF\u2600-\u27BF\uFE0F\u200D\U000E0020-\U000E007F]+'
 )
 
+# Strip <think>...</think> reasoning blocks before TTS — models with
+# /reasoning show enabled produce think blocks that shouldn't be spoken.
+_THINK_BLOCK = re.compile(r'<think[\s>].*?</think>', flags=re.DOTALL)
+
 
 def _strip_markdown_for_tts(text: str) -> str:
-    """Remove markdown formatting (and emoji) that shouldn't be spoken aloud."""
+    """Remove markdown, think blocks, and emoji that shouldn't be spoken."""
+    text = _THINK_BLOCK.sub(' ', text)
     text = _MD_CODE_BLOCK.sub(' ', text)
     text = _MD_LINK.sub(r'\1', text)
     text = _MD_URL.sub('', text)