Please address the comments from this code review:

## Overall Comments
- The legacy migration logic hardcodes `20` as the old timeout; consider introducing a `LEGACY_MIMO_TTS_TIMEOUT = 20` constant so the comparison stays in sync with any future changes and makes the intent clearer.
- The tests manually call `asyncio.run(provider.terminate())` in multiple places; factoring this into a small helper or fixture would reduce duplication and make it easier to adjust the termination behavior later.

## Individual Comments

### Comment 1
<location path="astrbot/core/provider/sources/mimo_tts_api_source.py" line_range="41" />
<code_context>
+            provider_config.get("timeout", DEFAULT_MIMO_TTS_TIMEOUT)
+        )
+        # MiMo TTS often needs longer than the legacy 20s default for large outputs.
+        if timeout in (None, 20):
+            timeout = DEFAULT_MIMO_TTS_TIMEOUT
+        self.timeout = timeout
</code_context>
<issue_to_address>
**suggestion:** Using `20` as a magic number here makes the migration behavior harder to follow.

This condition bakes in `20` as a special legacy value tied to the old default. To improve maintainability, consider naming this value (e.g., `LEGACY_MIMO_TTS_TIMEOUT`) or deriving the check from the raw config/legacy default instead, so behavior stays clear if defaults change later.

Suggested implementation:

```python
        timeout = normalize_timeout(
            provider_config.get("timeout", DEFAULT_MIMO_TTS_TIMEOUT)
        )
        # MiMo TTS often needs longer than the legacy default for large outputs.
        # Treat `LEGACY_MIMO_TTS_TIMEOUT` as a special value so behavior stays clear if defaults change.
        if timeout in (None, LEGACY_MIMO_TTS_TIMEOUT):
            timeout = DEFAULT_MIMO_TTS_TIMEOUT
        self.timeout = timeout

```

To fully implement the change and avoid the magic number, you should also:

1. Define a `LEGACY_MIMO_TTS_TIMEOUT` constant near the other MiMo-related defaults in `mimo_tts_api_source.py`, e.g. close to where `DEFAULT_MIMO_TTS_TIMEOUT` is declared:

   ```python
   LEGACY_MIMO_TTS_TIMEOUT = 20  # seconds; previous MiMo TTS timeout default
   DEFAULT_MIMO_TTS_TIMEOUT = ...
   ```

   (Preserve the existing value and placement of `DEFAULT_MIMO_TTS_TIMEOUT`; only insert the new constant above or nearby.)

2. If this module is imported elsewhere via `*` or re-exported, no further changes are needed, since the constant is only used internally here.
</issue_to_address>

### Comment 2
<location path="tests/test_mimo_api_sources.py" line_range="139-144" />
<code_context>
     }


+def test_mimo_tts_raises_legacy_default_timeout_to_60_seconds():
+    provider = _make_tts_provider({"timeout": "20"})
+    try:
+        assert provider.timeout == 60
+    finally:
+        asyncio.run(provider.terminate())
+
+
</code_context>
<issue_to_address>
**suggestion (testing):** Add coverage for legacy timeout expressed as an integer and missing/None timeout values

This only exercises the legacy timeout when given as a string. Since `provider_config.get("timeout", ...)` may return an `int` or `str` and the migration logic checks `timeout in (None, 20)` after `normalize_timeout`, please also add tests for `{"timeout": 20}`, `{"timeout": None}`, and (if not already covered) the case where `timeout` is omitted. That will validate the migration behavior across all legacy configurations.
</issue_to_address>