Unsafe Recovery: Add tombstoned tiflash learners into DemoteFailedVoters (#9554)

[ti-chi-bot]

This is an automated cherry-pick of #9554

What problem does this PR solve?

Issue Number: Close #9624

What is changed and how does it work?

Remove tombtstoned TiFlash learners in DemoteFailedVoters phase

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.

[v01dstar]

/retest

[v01dstar]

/test all

[v01dstar]

/retest-required

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 77.67%. Comparing base (0557634) to head (893b755). :warning: Report is 5 commits behind head on release-8.5.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-8.5    #9830      +/-   ##
===============================================
+ Coverage        75.87%   77.67%   +1.80%     
===============================================
  Files              467      467              
  Lines            73920    62369   -11551     
===============================================
- Hits             56088    48448    -7640     
+ Misses           14272    10347    -3925     
- Partials          3560     3574      +14     

Flag	Coverage Δ
unittests	77.67% <100.00%> (+1.80%)	:arrow_up:

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[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JmPotato, nolouch

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• ~~OWNERS~~ [JmPotato,nolouch]

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[ti-chi-bot[bot]]

[LGTM Timeline notifier]

Timeline:

• 2025-12-02 22:59:56.633732544 +0000 UTC m=+390741.447510106: :ballot_box_with_check: agreed by nolouch.
• 2025-12-03 06:24:33.811263609 +0000 UTC m=+417418.625041171: :ballot_box_with_check: agreed by JmPotato.