Gate named-blob 404->503 translation behind a config kill switch

[abha-mutalik]

Summary

PR #3234 (AMBRY-14247) changed the named-blob GET path to translate BlobDoesNotExist -> AmbryUnavailable (HTTP 503) when the named-blob metadata row exists but every storage replica returns BlobNotFound. The PR's own risk note flags potential retry-storm amplification on the inconsistency path, since the implicit 404 circuit-breaker is removed.

This adds a kill switch so the behavior can be disabled per-cluster without a code rollback.

• New config router.named.blob.translate.not.found.to.unavailable.enabled (default true, preserves current behavior).
• When false, NonBlockingRouter.translateNamedBlobMissingInStorage returns the original BlobDoesNotExist unchanged.
• The observability counter namedBlobMetadataExistsButStorageNotFoundCount still increments when the config is disabled so the underlying event rate remains visible during/after a rollback.

Testing Done

• Added testNamedBlobMissingInStorageNotTranslatedWhenConfigDisabled which sets the config to false, drives storage to BlobNotFound, and asserts the surfaced error is BlobDoesNotExist while the counter still increments.
• Overloaded setUpNamedBlobAndPut to accept custom Properties; existing call sites unchanged.
• Existing three named-blob translation tests still pass under default config (true).
• Ran the full parameterized suite:

  ./gradlew :ambry-router:test --tests "com.github.ambry.router.NonBlockingRouterTest.testNamedBlob*"
  

  24 tests passed (4 parameterized configs x 6 tests).

Risk

Default is unchanged behavior, so this is a no-op rollout. Setting the new flag to false restores the pre-#3234 behavior (404 surfaces to clients) on the inconsistency path only; the metric remains visible either way.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.23%. Comparing base (52ba813) to head (193fd52).
⚠️ Report is 392 commits behind head on master.

Files with missing lines	Patch %	Lines
...ava/com/github/ambry/router/NonBlockingRouter.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3264       +/-   ##
=============================================
- Coverage     64.24%   51.23%   -13.01%     
+ Complexity    10398     8678     -1720     
=============================================
  Files           840      931       +91     
  Lines         71755    79544     +7789     
  Branches       8611     9526      +915     
=============================================
- Hits          46099    40757     -5342     
- Misses        23004    35409    +12405     
- Partials       2652     3378      +726     

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[SophieGuo410]

why don't we just use the config to gate the translateNamedBlobMissingInStorage method?

[abha-mutalik]

Added the clause here so that the metric on the above line namedBlobMetadataExistsButStorageNotFoundCount still increments when the config is disabled, so that we still have insight into it whenever the metadata-vs-storage divergence happens.