[opt](cloud) Enable compaction on new tablets during schema change queuing

[Yukang-Lian]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Summary

In cloud mode, schema change (SC) tasks for multi-tablet tables are submitted to a thread pool with limited workers. Tablets waiting in the queue cannot do compaction because their alter_version is unset (-1), which causes rowset
accumulation during long queue waits.

This PR sets alter_version = V0 (from FE request) on new tablets at task pre-submit time (before enqueue), so that queued new tablets can perform cumulative compaction on double-write data (versions > V0) while waiting.

Changes

BE only — no FE or meta-service changes.

• task_worker_pool.h/cpp: Add pre_submit_callback to TaskWorkerPool, invoked synchronously before task enters the thread pool queue.
• task_worker_pool.cpp: Implement set_alter_version_before_enqueue() — sets alter_version = V0 on both base and new tablets before enqueue. Add unconditional alter_version cleanup in alter_cloud_tablet_callback().
• agent_server.cpp: Wire up set_alter_version_before_enqueue as pre_submit_callback for the ALTER worker pool.

Key Design Points

• V0 (partition visible version from FE) is set at pre-submit time to enable compaction during queue wait.
• V1 (base_max_version) is still determined at SC execution time for rowset conversion alignment — unchanged behavior.
• SC commit atomically replaces rowsets [2, V1] on the new tablet, so any compaction results in (V0, V1] during queue time are harmlessly discarded.
• Cleanup order: alter_version is reset to -1 before remove_task_info to prevent race with same-signature re-submission.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label