refactor(client): Improve robustness and fix mypy issues in client module

This commit introduces a series of refactorings and fixes to the async client module to improve its robustness, maintainability, and type safety.

Key changes include:
- Replaced brittle msgpack DataFrame serialization with the robust Arrow IPC format in the caching layer.
- Simplified cache eviction logic by removing redundant manual TTL management and relying on cachetools built-in functionality.
- Consolidated the API for fetching open positions by deprecating `get_positions` in favor of the more consistent `search_open_positions`.
- Removed misleading placeholder values from the `get_health_status` method to ensure it returns only accurately tracked metrics.
- Hardened the parsing of instrument contract IDs with a more resilient regex-based approach.
- Resolved all mypy type-checking errors across the client module, including issues with untyped libraries and mixin protocols.

Author: TexasCoding

src/project_x_py/client/base.py

@@ -115,7 +115,7 @@ def __init__(
 
     async def __aenter__(self) -> "ProjectXBase":
         """Async context manager entry."""
-        self._client = await self._create_client()
+        self._client = await self._create_client()  # type: ignore[misc]
         return self
 
     async def __aexit__(self, exc_type: Any, exc_val: Any, exc_tb: Any) -> None: