server: check target PD readiness before leader transfer

[HunDunDM]

Enhancement Task

PD currently supports /pd/api/v2/ready to indicate whether a PD member has finished the initial region loading process and is ready for leader promotion. However, some PD leader transfer paths may still transfer embedded etcd leadership to a target PD before that target has finished loading regions.

This can make a not-yet-ready PD become the PD leader, which may affect leader promotion behavior after restart or during recovery-sensitive operations.

Enhance PD leader transfer logic to check the target PD readiness before selected leader transfer paths call MoveEtcdLeader.

Scope:

• Add an opt-in target readiness check for MoveEtcdLeader.
• Apply the check to manual leader transfer APIs:
  • /pd/api/v1/leader/transfer/{next_leader}
  • /pd/api/v1/leader/resign
• Apply the check to automatic leader transfer by leader priority.
• Do not apply the check to recovery or fallback paths such as PD leader lost recovery and frequent campaign protection.
• Before calling /pd/api/v2/ready, request the target PD /pd/api/v1/version.
• If the target PD version does not support /ready, treat the target as ready after the version request succeeds for compatibility.
• Keep existing management API behavior unchanged. Do not add a force option or change request/response formats.

Expected behavior:

• Manual transfer to an unready target PD should fail before moving etcd leadership.
• Manual resign should skip unready candidates and choose a ready candidate when available.
• Manual resign should fail if no ready candidate exists.
• Priority-based leader transfer should skip the current round when the higher-priority target is not ready.
• Recovery paths should keep the existing behavior and should not be blocked by readiness checks.