Fix flaky rollup test by stopping jobs before index cleanup

[bowenlan-amzn]

Example failure: https://github.com/opensearch-project/index-management/actions/runs/19418611760/job/55551674679?pr=1455

The test was flaky because rollup job coroutines continued running after test cleanup, causing a race condition where the config index was recreated with incorrect dynamic mappings.

Root cause:

• Tests create enabled rollups -> JobScheduler schedules them
• RollupRunner.runJob() launches coroutines via launch {}
• Test @After wipes indices
• BUT coroutines are still running in background
• Coroutines write metadata AFTER indices wiped
• Index auto-creates with wrong dynamic mappings (long vs date)
• Next test fails with mapping conflict on rollup_metadata.continuous.next_window_end_time

Related Issues

Resolves #90

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.43%. Comparing base (ffe568f) to head (d383ebc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1530      +/-   ##
==========================================
+ Coverage   76.39%   76.43%   +0.03%     
==========================================
  Files         381      381              
  Lines       18058    18058              
  Branches     2501     2501              
==========================================
+ Hits        13796    13803       +7     
+ Misses       2991     2987       -4     
+ Partials     1271     1268       -3     

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[tandonks]

LGTM