Fix exception masking in FileSystems.matchSingleFileSpec

[yquaziii]

Fixes #40076

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Description:
This PR fixes a critical observability gap in FileSystems.matchSingleFileSpec.

Previously, if the underlying file system (such as GCS or HDFS) encountered an error and returned a MatchResult with an ERROR status, the actual root-cause exception (e.g., HTTP 403 Forbidden or 500 Internal Server Error) was swallowed by the SDK.

This change extracts the inner IOException from the failed MatchResult's .metadata() method and correctly chains it as the cause in the newly thrown IOException. This ensures the actual failure reason is exposed in the worker logs.

A unit test has been added to FileSystemsTest.java to verify the exception chaining behavior.

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[github-actions]

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R: @kennknowles for label java.

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[kennknowles]

Is there a way to test this without mocking - e.g. by setting up a local filesystem to result in this error, or using an in-memory fake for a blobstore?