fix: scope API V3 GC safe points by namespace and keyspace

[disksing]

API V3 TransactionClient::gc previously resolved locks within its namespace/keyspace but advanced the global PD safe point. Route these updates through UpdateGCSafePointV2 with the complete identity carried per request through the PD client and retry layers.

Other client modes retain the global RPC and the existing boolean result. Scoped failures never fall back to a global update, including PD implementations that do not support scoped GC.

Validation: Rust 1.93.0 formatting checks, all 145 library tests, and all-targets clippy with warnings denied passed on Linux. New gRPC tests cover two namespaces sharing a keyspace ID, legacy modes, a newer safe point, PD header errors, and an unsupported scoped RPC.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign imtbkcat for approval. For more information see the Code Review Process.
Please ensure that each of them provides their approval before proceeding.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

🗂️ Base branches to auto review (2)

• release-*
• tikv-*

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

• Configuration used: Organization UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: b8e9bbc0-0750-42d0-ba48-eeabf018294b

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}