fix(CDC): append chunks when downloading path-backed outputs

[tyler-french]

Fixes #29544

Using --remote_download_outputs=toplevel with --experimental_remote_cache_chunking and --disk_cache can cause Bazel to materialize a top-level output as a single CDC chunk instead of the full blob. This happens because the chunked downloader passes the final path-backed output stream into per-chunk downloads, letting DiskCacheClient's MaybePathBacked fast path copy each cached chunk directly over the final output path.

This PR fixes it by fetching each chunk through CombinedCache as bytes and then appending those bytes to the final output stream, preserving disk cache policy/write-through behavior for chunks while keeping the path-backed copy optimization for whole-blob downloads.

This is unintentionally fixed in #29341
But I think this might be worth a patch release in 9.1.1 and/or 8.7.1

[tjgq]

@bazel-io 9.1.1

[iancha1992]

@tyler-french 9.1.0 is already released. Let's also wait until it is merged to the master branch first before we merge this to the release branch because we prefer the cherry-pick bot to do the work. I will reopen this if there's a merge conflict and help is needed. Thank you!

[tyler-french]

@tyler-french 9.1.0 is already released. Let's also wait until it is merged to the master branch first before we merge this to the release branch because we prefer the cherry-pick bot to do the work. I will reopen this if there's a merge conflict and help is needed. Thank you!

EDIT: Thanks, just realized #29341 IS going into 9.1.1. Thank you!