stats: add defensive guard for interval calculation (#65889)

[ti-chi-bot]

This is an automated cherry-pick of #65889

What problem does this PR solve?

Issue Number: close #65815

Problem Summary:
Negative durations from mysql.analyze_jobs (e.g. clock skew/bad records) can happen and should be handled defensively without skewing retry logic.

What changed and how does it work?

Based on #65821.

• Document why negative average durations return NoRecord.
• Add unit tests for negative finished-job durations.

Check List

Tests

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

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (release-8.5@30b8f75). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             release-8.5     #66009   +/-   ##
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  Coverage               ?   38.1727%           
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  Files                  ?       1731           
  Lines                  ?     635430           
  Branches               ?          0           
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  Hits                   ?     242561           
  Misses                 ?     368457           
  Partials               ?      24412           

Flag	Coverage Δ
integration	38.1727% <0.0000%> (?)

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Components	Coverage Δ
dumpling	∅ <0.0000%> (?)
parser	∅ <0.0000%> (?)
br	36.0697% <0.0000%> (?)

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Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	1330d14	link	true	/test unit-test
idc-jenkins-ci-tidb/build	1330d14	link	true	/test build

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