test(fileservice): cover streaming disk cache edge cases

[LeftHandCold]

Summary

• add coverage for range reads bypassing a prefetch full-object io merge
• add streaming full-object disk cache coverage for read-to-end and unexpected EOF cases
• add coverage for WriterForRead, ReadCloserForRead, and empty ranges being excluded from streaming disk cache fill

Tests

• GOCACHE=/private/tmp/mo-gocache-fileservice-ut go test -mod=readonly ./pkg/fileservice -run 'TestS3FSRangeReadSkipsPrefetchFullObjectIOMerge|TestS3FSReadFullObjectToDiskCacheStreamingReadToEnd|TestS3FSReadFullObjectToDiskCacheStreamingUnexpectedEOF|TestS3FSShouldStreamFullObjectToDiskCacheExclusions|TestS3FSReadFullObjectToDiskCacheStreamingReturnsReaderError' -count=1
• GOCACHE=/private/tmp/mo-gocache-fileservice-ut go test -mod=readonly ./pkg/fileservice -count=1

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[Copilot]

Pull request overview

Adds new test coverage for streaming disk cache edge cases in pkg/fileservice/s3_fs_test.go, including range reads bypassing prefetch full-object IO merge, read-to-end (-1 size) handling, unexpected EOF, and exclusions in shouldStreamFullObjectToDiskCache for empty ranges, WriterForRead, and ReadCloserForRead.

Changes:

• Add TestS3FSRangeReadSkipsPrefetchFullObjectIOMerge verifying range reads don't block on in-flight prefetch merges
• Add streaming disk cache tests for read-to-end and unexpected EOF behaviors
• Add TestS3FSShouldStreamFullObjectToDiskCacheExclusions covering exclusion conditions