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/web_tools.py

@@ -73,7 +73,7 @@ def _get_backend() -> str:
     Falls back to whichever API key is present for users who configured
     keys manually without running setup.
     """
-    configured = _load_web_config().get("backend", "").lower().strip()
+    configured = (_load_web_config().get("backend") or "").lower().strip()
     if configured in ("parallel", "firecrawl", "tavily"):
         return configured