Fix all mypy type errors and flaky embedding test

- Fix 34 mypy errors across 17 files with type annotations and casts
- Add type: ignore comments for legitimate dynamic patterns
- Handle None operands with (value or 0) pattern
- Cast return values to proper types (int, float, str, bool)
- Add EstimatingTokenCounter imports where needed
- Use getattr() for potentially missing attributes
- Fix flaky test_paraphrase_match with more distinct semantic examples
- Add mlx to mypy ignore list (broken third-party stubs)

headroom/ccr/mcp_server.py

@@ -53,7 +53,7 @@ try:
     HTTPX_AVAILABLE = True
 except ImportError:
     HTTPX_AVAILABLE = False
-    httpx = None
+    httpx = None  # type: ignore[assignment]
 
 from .tool_injection import CCR_TOOL_NAME
 

headroom/integrations/langchain.py

@@ -55,8 +55,8 @@ except ImportError:
     LANGCHAIN_AVAILABLE = False
     BaseChatModel = object
     BaseCallbackHandler = object
-    Field = lambda **kwargs: None  # noqa: E731
-    PrivateAttr = lambda **kwargs: None  # noqa: E731
+    Field = lambda **kwargs: None  # type: ignore[assignment]  # noqa: E731
+    PrivateAttr = lambda **kwargs: None  # type: ignore[assignment]  # noqa: E731
 
 from headroom import HeadroomConfig, HeadroomMode
 from headroom.providers import OpenAIProvider