fix: fail-fast import guard for ArrowSerializer when pyarrow missing

[27Bslash6]

Summary

Fixes a timing mismatch in the import guard that caused HAS_ARROW_SERIALIZER to be incorrectly set to True even when pyarrow wasn't installed.

The Bug

# auto_serializer.py
try:
    from .arrow_serializer import ArrowSerializer
    HAS_ARROW_SERIALIZER = True  # ← This succeeded!
except ImportError:
    HAS_ARROW_SERIALIZER = False

# arrow_serializer.py  
try:
    import pyarrow as pa
    HAS_PYARROW = True
except ImportError:
    HAS_PYARROW = False  # ← pyarrow missing, but class still defined

class ArrowSerializer:  # ← Class is importable even without pyarrow!
    def __init__(self):
        if not HAS_PYARROW:
            raise ImportError(...)  # ← Too late - guard already passed

The Fix

Fail-fast at module level so the import guard catches it:

# arrow_serializer.py
try:
    import pyarrow as pa
except ImportError as e:
    raise ImportError("pyarrow is not installed...") from e  # Fail here, not in __init__

Test plan

• make quick-check passes
• All pre-commit hooks pass

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/cachekit/serializers/arrow_serializer.py	0.00%	2 Missing ⚠️

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