Add opt-in all-version purge to named-blob cleanup

[zichengl]

Summary

1. Add an opt-in all-version purge mode to the named-blob cleaner, including latest READY and recent IN_PROGRESS versions. Existing constructors keep purge disabled.
2. Use ascending primary-key order for MySQL purge pages, retaining an inclusive last-name cursor so a single name can span multiple pages.
3. Add a caller-owned eligibility predicate, bounded first-page dry-run preview, counters, and failure-safe paging. Exclusions remain authoritative and container lifecycle status is unchanged.

Durability risk: high when enabled. Purge intentionally deletes live/latest versions. Callers must explicitly approve each container, fence source reads/writes, and verify that the source data is no longer needed. Eligibility is checked before each page and delete, but revocation cannot cancel an already-issued deletion. Deletion or metadata failures stop purge; subsequent runs restart the sweep. Repeated versions after metadata cleanup also stop the sweep, and purge never skips names after query kills.

This draft proposes only the generic OSS mechanism. Deployment configuration, migration/completeness checks, composite-database delegation, and verification of composite-router delete routing remain integration responsibilities. No rollout is included. Expired/already-deleted mappings retain their existing cleanup paths. Primary-key ordering avoids the mixed-order filesort but does not guarantee bounded latency when many index entries are filtered.

Please review the API/runner policy first, then the MySQL paging behavior. Live SQL validation remains pending.

Testing Done

Using JDK 17:

Command	Result
./gradlew --no-daemon --console=plain -PdisableShipkit :ambry-frontend:test --tests com.github.ambry.frontend.NamedBlobsCleanupRunnerTest :ambry-named-mysql:test --tests com.github.ambry.named.MySqlNamedBlobDbTest :ambry-named-mysql:compileIntTestJava	Passed: 24 cleaner tests and 19 database unit tests; integration tests compile.
git diff --check	Passed.
./gradlew --no-daemon --console=plain -PdisableShipkit :ambry-named-mysql:intTest --tests 'com.github.ambry.named.MySqlNamedBlobDbIntegrationTest.testPurge*'	Blocked: local MySQL connection refused. Paging and EXPLAIN assertions are not yet verified against a running database.

Full build/full test suite not run.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.01%. Comparing base (52ba813) to head (e13e1ec).
⚠️ Report is 424 commits behind head on master.

Files with missing lines	Patch %	Lines
...github/ambry/frontend/NamedBlobsCleanupRunner.java	0.00%	48 Missing ⚠️
.../java/com/github/ambry/named/MySqlNamedBlobDb.java	0.00%	14 Missing ⚠️
.../main/java/com/github/ambry/named/NamedBlobDb.java	0.00%	5 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (52ba813) and HEAD (e13e1ec). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (52ba813)	HEAD (e13e1ec)
	3	2

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3305       +/-   ##
=============================================
- Coverage     64.24%   38.01%   -26.24%     
+ Complexity    10398     6486     -3912     
=============================================
  Files           840      939       +99     
  Lines         71755    80881     +9126     
  Branches       8611     9768     +1157     
=============================================
- Hits          46099    30743    -15356     
- Misses        23004    47651    +24647     
+ Partials       2652     2487      -165     

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