gc: Add a temporary solution to make service safe point compatible with the new txn safe point mechanism

[MyonKeminta]

What problem does this PR solve?

Issue Number: ref #8978

The UpdateServiceGCSafePoint(V2) methods are going to be deprecated and replaced by a compatibility layer over the GCStatesManager, which is expected to be done in #9292. However it's not sure when #9292 can be merged. Now, there still exist usages of UpdateServiceGCSafePoint(V2), but TiDB won't register the service safe point of "gc_worker" anymore. This causes that when other components tries to register their service safe points, the "gc_worker"'s service safe point may be automatically initialized as 0, allowing data access that breaks the constraint of txn safe point.

This PR is a temporary solution to this problem, and should be finally cleaned up in #9292. This PR makes the service safe point of "gc_worker" always be initalized to the same value as the current txn safe point.

What is changed and how does it work?

This PR makes the initialization of the service safe point of gc_worker to consider the txn safe point.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
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[MyonKeminta]

/retest

[codecov]

Codecov Report

Attention: Patch coverage is 59.09091% with 9 lines in your changes missing coverage. Please review.

Project coverage is 76.20%. Comparing base (ecb09cc) to head (69fcf70).
Report is 1 commits behind head on master.

❌ Your patch status has failed because the patch coverage (59.09%) is below the target coverage (74.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9407      +/-   ##
==========================================
+ Coverage   76.16%   76.20%   +0.04%     
==========================================
  Files         478      478              
  Lines       74782    74794      +12     
==========================================
+ Hits        56954    56999      +45     
+ Misses      14296    14276      -20     
+ Partials     3532     3519      -13     

Flag	Coverage Δ
unittests	76.20% <59.09%> (+0.04%)	⬆️

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: okJiang, rleungx

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Details

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• OWNERS [rleungx]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-06-13 08:08:46.587854269 +0000 UTC m=+601704.816169530: ☑️ agreed by rleungx.
• 2025-06-13 08:36:12.589831427 +0000 UTC m=+603350.818146691: ☑️ agreed by okJiang.

[MyonKeminta]

/retest

[MyonKeminta]

/retest

[MyonKeminta]

/retest

[MyonKeminta]

/retest

[MyonKeminta]

/retest