Log and meter S3 batch-delete request-body parse failures

[snalli]

JIRA: ACTIONITEM-16798

Summary

S3BatchDeleteHandler.parseRequestBodyAndDeleteCallback catches every
Exception thrown while validating the request body and maps it to a
generic MalformedRequestBody S3 error. The caught exception is never
logged, no metric is incremented, and the callback is invoked with a
null exception argument — so a genuine server-side bug (NPE, IO
failure, deserializer crash, etc.) is operationally indistinguishable
from a client sending malformed XML.

This change:

• Adds FrontendMetrics.s3BatchDeleteRequestParseError (Counter).
• In the catch block, increments the counter and logs the exception at
  WARN with logger.warn("Failed to parse S3 batch-delete request body, returning malformed-body error", e).
• Leaves the S3 protocol response (200 OK with XML error body) unchanged
  — that's a protocol requirement.

Testing Done

• S3BatchDeleteHandlerTest.malformedXMLRequestTest now also asserts
  the parse-error counter increments by exactly 1 when a malformed body
  fires the catch.
• All 6 tests in S3BatchDeleteHandlerTest pass:
  ./gradlew :ambry-frontend:test --tests "com.github.ambry.frontend.S3BatchDeleteHandlerTest"
• :ambry-frontend:compileJava and :ambry-frontend:compileTestJava are clean.

Durability risk analysis

No write/metadata/ordering paths touched. This is a pure observability
change in the request-validation path of an S3-compat batch delete
handler. The on-the-wire response is byte-for-byte identical to before.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.22%. Comparing base (52ba813) to head (803f947).
⚠️ Report is 384 commits behind head on master.

Files with missing lines	Patch %	Lines
...github/ambry/frontend/s3/S3BatchDeleteHandler.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3255       +/-   ##
=============================================
- Coverage     64.24%   51.22%   -13.03%     
+ Complexity    10398     8666     -1732     
=============================================
  Files           840      931       +91     
  Lines         71755    79486     +7731     
  Branches       8611     9515      +904     
=============================================
- Hits          46099    40716     -5383     
- Misses        23004    35380    +12376     
- Partials       2652     3390      +738     

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