fix: improve shard publish error logging

[brendan-kellam]

Summary

• log the temporary and final paths, concrete OS error type, and filesystem state when publishing a shard fails
• preserve the first build error so a later stale-shard cleanup failure does not hide the original rename failure
• add a regression test covering simultaneous publish and cleanup failures

Motivation

We observed an index build report only remove ...: no such file or directory after logging that a shard had finished. The actual preceding temp-to-final rename error was not logged and was subsequently overwritten, obscuring whether the failure came from SMB rename semantics or concurrent cleanup.

Testing

• go test ./...

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Note

Low Risk
Localized observability and error-handling change in index Finish; behavior on success is unchanged, with a regression test for the failure path.

Overview
Builder.Finish now keeps the first failure during shard publish and cleanup instead of letting a later step replace it—so a misleading cleanup error (e.g. remove: no such file) no longer hides a failed temp→final rename.

On rename failure it logs temp/final paths, the concrete OS error type, and fileState (mode, size, modtime) for both paths, and wraps the error as publishing shard: rename .... Other late-stage errors (old path discovery, tombstones, removing stale shards) go through the same recordBuildError helper with clearer messages where applicable.

Adds TestBuilderFinishPreservesShardPublishError when the final shard path is a non-empty directory so publish and cleanup both fail but the returned error still mentions publishing.

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