fix(acp): replace global _ACP_CACHE_DIR with @lru_cache

[github-actions]

Summary

Replaces module-level global state for caching the ACP cache directory with functools.lru_cache decorator. This is a cleaner pattern that avoids mutable module-level state while maintaining the same lazy initialization behavior.

Changes

• Removed global _ACP_CACHE_DIR variable
• Added @lru_cache(maxsize=1) decorator to get_acp_cache_dir() function
• Simplified function body by removing global state management

Before:

_ACP_CACHE_DIR: Path | None = None

def get_acp_cache_dir() -> Path:
    global _ACP_CACHE_DIR
    if _ACP_CACHE_DIR is None:
        _ACP_CACHE_DIR = Path.home() / ".openhands" / "cache" / "acp"
        _ACP_CACHE_DIR.mkdir(parents=True, exist_ok=True)
    return _ACP_CACHE_DIR

After:

@lru_cache(maxsize=1)
def get_acp_cache_dir() -> Path:
    cache_dir = Path.home() / ".openhands" / "cache" / "acp"
    cache_dir.mkdir(parents=True, exist_ok=True)
    return cache_dir

Related Issue

Addresses findings from #557

Testing

• Linting passes (make lint)
• Tests pass (make test) - 1261 tests passed

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This PR was automatically generated by the Code Quality Report workflow.