chunker: fix single-file operations over union remotes - #9897

What does this change do?

Fix single-file listing, copying and moving when chunker wraps a union whose chunks are distributed across upstreams. Opening the union at the metadata file (or first chunk) can restrict it to the upstream containing that file, leaving other chunks inaccessible even though directory reads work.

Detect a file through its cached parent and keep that complete parent as chunker's base. Configure hashing, transaction mode and features from the final base. This also handles metadata-free files and preserves the standard cache lifecycle and path handling. The extra parent-object probe is confined to chunker; ordinary union single-file behavior is unchanged.

Add eight regression cases covering both metadata formats, configured and nested roots, metadata-only upstreams, distributed chunks, repeated file opens and directory reads. All eight fail against the unmodified production code and pass with this change.

This contribution was implemented and tested with OpenAI Codex assistance.

Linked issue

Fixes #7955

The issue includes the maintainer's reproduction and analysis.

For new or changed backends

• go build: passed.
• go test ./backend/chunker ./backend/union/... -count=1: passed.
• make quicktest: passed as an unprivileged user. FTP loopback tests ran in a private network namespace to avoid host Kubernetes NodePort conflicts.
• go run ./fstest/test_all -backends chunker,union -tests backend -config <local-test-config> -n 2 -maxtries 1: 4/4 passed, against chunker/union, chunker/local, metadata-free chunker/local and ordinary union, all backed by temporary local directories.
• The additional full test_all matrix encounters an existing bisync setup failure on chunker/union: sync.CopyDir returns object not found while setting up path1. This is reproduced on unmodified master ef6968730fc5582aa2a1db028440f3acc48b4e27 with the same configuration and unprivileged user using go test ./cmd/bisync -run '^TestBisyncRemoteLocal$/^all_changed$' -remote TestChunkerUnion: -count=1 -v. The extra full-matrix attempt was stopped after confirming this baseline failure; a complete full-matrix pass is not claimed.
• CLI reproduction: with a naturally distributed 5 MiB file, directory copying succeeds before the fix while single-file listing is empty and copyto/moveto fail. All commands pass after the fix. An independent run with 5 MiB + 137 bytes of random data passes listing, copy, move and renamed-file copy with matching SHA-256 hashes and removal of the old remote file/chunks.
• golangci-lint run --new-from-rev ef6968730fc5582aa2a1db028440f3acc48b4e27: 0 issues. Full lint reports the same existing revive warning at fs/log.go:17 on both the unmodified base and this patch.

The integration remotes use existing backends and temporary local storage; no cloud account or new backend is involved.

Checklist

• This change is trivial OR it has been discussed and agreed in the linked issue.
• I have read the contribution guidelines.
• (If I used AI tools to help write this code) I have read and understood the AI-assisted contributions guidance, and I have tested and take ownership of this change myself.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate. (No configuration or user-facing documentation change is needed.)
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend and if submitting a new backend can provide a test account for the integration tester - see CONTRIBUTING.md.
• This Pull Request is ready for review.