copyurl: honor --size-only for existing destinations - #9896

What does this change do?

Make copyurl --size-only skip an existing destination when its size matches the HTTP response's known Content-Length. With an 8-byte destination containing original and an 8-byte response containing replaced, the destination is currently overwritten despite --size-only; this change preserves it and reports 0 transferred bytes.

The existing GET request still obtains the response headers and filename; the matching body is not passed to the upload path. Unknown lengths still copy normally, and --no-clobber retains precedence. The help text documents this behavior.

Validation:

• A real HTTP server plus the local backend reproduces the overwrite before the fix. Regression coverage checks matching/different/unknown sizes, missing destinations, and --no-clobber precedence.
• Built both upstream and fixed CLI binaries and ran the same HTTP reproduction: before, 8 bytes transferred and content overwritten; after, 0 bytes transferred and content preserved.
• go build and go test ./fs/operations ./cmd/copyurl -count=1 passed.
• Full make quicktest passed (180 packages, no test filters). Ran as an unprivileged user with a matching CLI on PATH; only the FTP test executable used a loopback network namespace because its passive port range overlaps this host's Kubernetes NodePorts. No test source or host firewall changes were required.
• golangci-lint run ./fs/operations/... ./cmd/copyurl/...: 0 issues.

Linked issue

Fixes #9892

For new or changed backends

Not applicable.

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.
• All commit messages are in house style.
• (Backend changes only) test_all passes for this backend.
• This Pull Request is ready for review.

AI tools assisted implementation, review, and the local executions described above. The account owner's separate personal review/ownership attestation is left unchecked.