Fix NPE in NettyResponseChannel when root cause exception has null message

[crliao]

Problem & Solution Overview

CompletableFuture.orTimeout() constructs a TimeoutException with no message (new TimeoutException()). When this exception is the root cause of a RestServiceException, Utils.getRootCause(cause).getMessage() returns null, causing an NPE in NettyResponseChannel.getErrorResponse() at line 593.

This fires when shouldSendFailureReason returns true — i.e. when the request has SEND_FAILURE_REASON=true (set by NamedBlobPutHandler, S3MultipartUploadPartHandler, and UndeleteHandler). Any named blob PUT whose ID conversion times out will hit this path, logging a spurious NPE stack trace on top of the 503.

Fix: null-check the root cause message before using it. If null, errReason stays null and no FAILURE_REASON_HEADER is set — same behavior as when shouldSendFailureReason returns false.

Testing Done

• Added setFailureReasonNullMessageNoNpeTest in NettyResponseChannelTest: wraps a bare new TimeoutException() (no message) inside a RestServiceException with SEND_FAILURE_REASON=true, sends through embedded channel, asserts 503 response with no NPE and no FAILURE_REASON_HEADER.
• Existing setFailureReasonInResponseTest still passes.

Notes for Reviewers

Root cause of null message: CompletableFuture.orTimeout() JDK source calls new TimeoutException() with no string argument. The NPE only fires on the shouldSendFailureReason=true code path.

Author Checklist

• For significant code changes - code is LiX'ed and/or completely backwards compatible.
• For added/changed user-exposed functionality - appropriate documentation has been added.
• If necessary, new metrics and/or alerts have been added.
• If a high-risk change, I've articulated what to look out for in the sections above.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.24%. Comparing base (52ba813) to head (bff62f4).
⚠️ Report is 392 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/com/github/ambry/rest/NettyResponseChannel.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3263       +/-   ##
=============================================
- Coverage     64.24%   51.24%   -13.00%     
+ Complexity    10398     8687     -1711     
=============================================
  Files           840      931       +91     
  Lines         71755    79542     +7787     
  Branches       8611     9526      +915     
=============================================
- Hits          46099    40764     -5335     
- Misses        23004    35400    +12396     
- Partials       2652     3378      +726     

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[SophieGuo410]

will Utils.getRootCause(cause) return null

[crliao]

No — getRootCause(cause) cannot return null here. If the input is non-null, the loop starts with throwable = t (non-null) and only advances while throwable.getCause() != null; when the chain ends it returns the current throwable, still non-null. It only returns null when the input is null. At this call site, cause has already passed the instanceof RestServiceException check on the enclosing if, so it is guaranteed non-null.

[crliao]

at line 586 if (cause instanceof RestServiceException)