cluster: abort force scale-in when PD delete fails

[lhy1024]

What problem does this PR solve?

Close #2705

When tiup cluster scale-in --force scales in a TiKV instance, TiUP currently ignores errors from deleting the store/member in PD, then continues to stop/destroy the instance and remove it from the topology.

If PD rejects the delete-store request, for example because removing the TiKV would make the number of remaining up stores less than max-replicas, the store remains Up / Serving in PD while TiUP no longer shows the instance in tiup cluster display.

What is changed and how it works?

• Keep using the normal PD delete-store API for TiKV/TiFlash scale-in. This PR does not mark the store as physically destroyed and does not pass PD's force query.
• In the scale-in --force path, return the PD delete error immediately instead of warning and continuing.
• This prevents TiUP from stopping/destroying the instance and updating topology when PD has not accepted the store/member deletion.
• Add a unit test to ensure PDClient.DelStore does not send the force query parameter.

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

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

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

Fix an issue that `tiup cluster scale-in --force` could remove a TiKV instance from topology even when PD rejected deleting its store.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.14%. Comparing base (9bdae04) to head (77a4237).

Files with missing lines	Patch %	Lines
pkg/cluster/operation/scale_in.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2704      +/-   ##
==========================================
- Coverage   42.71%   42.14%   -0.57%     
==========================================
  Files         424      424              
  Lines       49744    47146    -2598     
==========================================
- Hits        21244    19868    -1376     
+ Misses      25803    24595    -1208     
+ Partials     2697     2683      -14     

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