[fix](cloud) skip stale tablet cache check for STOP_TOKEN to fix spurious schema change failure

[Hastyshell]

Problem

In cloud mode, schema change on MOW (Merge-on-Write) tables intermittently fails with:

task type: ALTER, status_code: INTERNAL_ERROR, status_message:
[(BE_IP)[INTERNAL_ERROR]failed to start tablet job:
meta_service_job.cpp could not perform compaction on expired tablet cache.
req_base_compaction_cnt=0, base_compaction_cnt=0,
req_cumulative_compaction_cnt=8, cumulative_compaction_cnt=9]

Root Cause

Schema change on a MOW table calls _process_delete_bitmap(), which registers a STOP_TOKEN compaction job via CloudCompactionStopToken::do_register(). The STOP_TOKEN is not a real compaction — it is a lock marker that blocks concurrent compactions during delete bitmap recalculation.

However, start_compaction_job() in the meta-service applies the stale tablet cache check unconditionally to all compaction types, including STOP_TOKEN. If a concurrent compaction on another BE node advances cumulative_compaction_cnt in the meta-service while the schema change BE still holds its old cached value, the STOP_TOKEN registration is rejected with STALE_TABLET_CACHE. This error propagates back to the FE as a fatal ALTER task failure.

Fix

Skip the stale tablet cache check when the compaction job type is STOP_TOKEN. Since STOP_TOKEN does not read or compact any rowsets, verifying the freshness of cached compaction counts is meaningless for it.

// Before
if (compaction.base_compaction_cnt() < stats.base_compaction_cnt() ||
    compaction.cumulative_compaction_cnt() < stats.cumulative_compaction_cnt()) {

// After
if (compaction.type() != TabletCompactionJobPB::STOP_TOKEN &&
    (compaction.base_compaction_cnt() < stats.base_compaction_cnt() ||
     compaction.cumulative_compaction_cnt() < stats.cumulative_compaction_cnt())) {

Testing

Added regression test StopTokenSkipsStaleTabletCacheCheck in cloud/test/meta_service_job_test.cpp that:

1. Sets up a tablet with cumulative_compaction_cnt=9 on the meta-service side
2. Verifies that a regular CUMULATIVE compaction with stale count=8 is still correctly rejected with STALE_TABLET_CACHE
3. Verifies that a STOP_TOKEN with the same stale count=8 succeeds with OK

[Thearas]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
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4. Which code was refactored and why was this part of the code refactored?
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[Hastyshell]

run buildall

[hello-stephen]

Cloud UT Coverage Report

Increment line coverage 100.00% (3/3) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	79.24% (1798/2269)
Line Coverage	64.46% (32278/50077)
Region Coverage	65.35% (16163/24732)
Branch Coverage	55.78% (8612/15438)