fix: return finalizer readiness errors

[immanuwell]

Related: #1550, #1562

retry.OnError(...) was already used in the finalizer readiness check, but the returned error got dropped. If the target Deployment hits ProgressDeadlineExceeded, finalization still keeps going anyway, kinda rough.

This stores that error and stops finalization until the target is ready. Added a regression test for the stuck Deployment case.

Repro

1. Create a Canary with revertOnDeletion: true.
2. Delete it while the target Deployment reports Progressing=False and Reason=ProgressDeadlineExceeded.
3. On main, finalize() returns nil and finalization completes.
4. With this patch, Flagger returns canary not ready during finalizing and retries later.

Tested with go test ./....

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 29.99%. Comparing base (61582f7) to head (2663b97).
⚠️ Report is 10 commits behind head on main.
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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1939      +/-   ##
==========================================
- Coverage   30.00%   29.99%   -0.01%     
==========================================
  Files         288      288              
  Lines       18455    18614     +159     
==========================================
+ Hits         5537     5584      +47     
- Misses      12189    12295     +106     
- Partials      729      735       +6     

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