feat(replication): structure active check results

[cxymds]

Related Issues

Closes rustfs/backlog#1417

Related: rustfs/backlog#1381, rustfs/backlog#1361, rustfs/backlog#1496

Summary of Changes

• Return a structured result for every configured replication target and every
  bucket, versioning, object-lock, put, delete-marker, version-delete, and
  cleanup phase.
• Disclose that the GET extension performs an active temporary mutation and
  publish the reserved probe namespace in the response contract.
• Allocate collision-resistant keys below
  .rustfs.sys/replication-check/, preflight exact object versions, and use a
  conditional put so user/concurrent objects are never overwritten or removed.
• Run cleanup after ambiguous writes and later phase failures, continue version
  deletion after delete-marker failures, and report cleanup failure separately.
• Bound and sanitize remote failures to structured safe codes without exposing
  endpoints, credentials, signed URLs, or raw server details.
• Preserve successful target results when another target fails and document the
  contract and operator implications.

Verification

• 108 replication-focused unit tests passed; 2 environment-dependent tests
  ignored.
• Real two-server replication extension suite: 5 passed, including successful
  cleanup with zero remaining target versions/delete markers and an object-lock
  mismatch that performed no mutation.
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• make pre-commit
• make pre-pr (all checks passed: 10,299 workspace tests and all doctests)
• cargo fmt --all --check
• git diff --check

Adversarial review verdicts:

• Security/redaction: PASS
• Correctness/state machine: PASS
• Concurrency/collision safety: PASS
• Cleanup/failure recovery: PASS
• API compatibility: PASS
• Test quality: PASS
• Operations/documentation: PASS

Impact

The replication-check success body changes from empty to structured JSON on
RustFS servers. The operation remains a signed S3 GET extension, but its active
temporary mutations are now explicit. Older clients that require an empty body
must be updated; rustfs/backlog#1496 tracks typed rendering plus legacy
empty-body compatibility.

Additional Notes

The reserved namespace and conditional preflight prevent this diagnostic from
deleting an object it did not create. Cleanup errors are deliberately returned
as failed structured outcomes even when the earlier probe phases succeeded.

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