fix: guard config.get() against YAML null values to prevent AttributeError (#3377)

dict.get(key, default) returns None — not the default — when the key IS
present but explicitly set to null/~ in YAML.  Calling .lower() on that
raises AttributeError.

Use (config.get(key) or fallback) so both missing keys and explicit nulls
coalesce to the intended default.

Files fixed:
- tools/tts_tool.py — _get_provider()
- tools/web_tools.py — _get_backend()
- tools/mcp_tool.py — MCPServerTask auth config
- trajectory_compressor.py — _detect_provider() and config loading

Co-authored-by: dieutx <dangtc94@gmail.com>

tools/tts_tool.py

@@ -102,7 +102,7 @@ def _load_tts_config() -> Dict[str, Any]:
 
 def _get_provider(tts_config: Dict[str, Any]) -> str:
     """Get the configured TTS provider name."""
-    return tts_config.get("provider", DEFAULT_PROVIDER).lower().strip()
+    return (tts_config.get("provider") or DEFAULT_PROVIDER).lower().strip()
 
 
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