Add defensive check to prevent double-wrapping in spec_from_file_location wrapper

[Copilot]

Addresses feedback from PR #77 to add a defensive check preventing double-wrapping of loaders when wrap_spec_from_file_location() processes specs with already-wrapped loaders.

Changes

• Added defensive check in wrap_spec_from_file_location(): Returns early if spec.loader is already a SlipcoverLoader, preventing nested wrapping in edge cases where an already-instrumented spec is passed through the wrapper
• Added unit test: test_wrap_spec_from_file_location_no_double_wrap() validates the defensive check by passing a wrapped loader as the loader parameter to spec_from_file_location()

The check follows the existing pattern for skipping AssertionRewritingHook and ExtensionFileLoader:

# Skip if already wrapped - prevent double-wrapping
if isinstance(spec.loader, SlipcoverLoader):
    return spec

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