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

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

tools/mcp_tool.py

@@ -797,7 +797,7 @@ class MCPServerTask:
         """
         self._config = config
         self.tool_timeout = config.get("timeout", _DEFAULT_TOOL_TIMEOUT)
-        self._auth_type = config.get("auth", "").lower().strip()
+        self._auth_type = (config.get("auth") or "").lower().strip()
 
         # Set up sampling handler if enabled and SDK types are available
         sampling_config = config.get("sampling", {})