transaction: Add "scan_locks" & "resolve_locks" interface for transaction client

[pingyu]

Changes

• Add public TransactionClient::scan_locks and TransactionClient::resolve_locks for externally-driven lock cleanup (src/transaction/client.rs:300, src/transaction/client.rs:322).
• Add keyspace encoding for Vec<LockInfo> so locks returned/truncated by scan_locks can be re-encoded before resolving (src/request/keyspace.rs:185).
• Reuse plan-layer gRPC classification (src/request/plan.rs:91) and harden ResolveLock retries with backoff + cache invalidation (src/transaction/lock.rs:145).
• Add coverage for retry + “resolve committed even if TTL not expired” (src/transaction/lock.rs:626, src/transaction/lock.rs:674) and an integration failpoint test for the new API (tests/failpoint_tests.rs:287).

Compare with Go implementation by LLM:

• Rust resolve_locks (src/transaction/lock.rs:51) follows the same non-async, non-pessimistic flow: CheckTxnStatus per txn, then ResolveLock per txn+region with per-txn commit-version + cleaned-region caching
  (mirrors Go’s cleanTxns).
• ResolveLock retry behavior (src/transaction/lock.rs:145) now matches Go’s resolveLock loop more closely: backoff-based retries, region-error handling via handle_region_error, plus region/store cache invalidation on leader/grpc issues.
• Expected deltas (out of scope per request): pessimistic-lock + async-commit resolving, resolve-lock-lite, and Rust returning “live locks + caller backoff” instead of Go’s TTL return.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: coocood, iosmanthus

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2026-01-21 05:53:33.306366025 +0000 UTC m=+559640.920322871: ☑️ agreed by coocood.
• 2026-01-21 09:55:54.507534085 +0000 UTC m=+574182.121490941: ☑️ agreed by iosmanthus.