xdsclient: fix panic on empty resource in ADS response

[Pranjali-2501]

This PR fixes an intermittent panic in xdsclient decodeResponse that occurs when processing an xDS ADS response containing an empty resource name, while doing fuzz testing.

RELEASE NOTES:

• xds: Fix a panic in the xDS client that can occur when processing an ADS response containing an empty resource name.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.11%. Comparing base (81c7924) to head (054d10a).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/xds/clients/xdsclient/channel.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8970      +/-   ##
==========================================
+ Coverage   82.67%   83.11%   +0.43%     
==========================================
  Files         410      411       +1     
  Lines       32573    32715     +142     
==========================================
+ Hits        26929    27190     +261     
- Misses       4055     4149      +94     
+ Partials     1589     1376     -213     

Files with missing lines	Coverage Δ
...nternal/xds/xdsclient/xdsresource/unmarshal_cds.go	88.70% <100.00%> (+0.36%)	⬆️
...nternal/xds/xdsclient/xdsresource/unmarshal_eds.go	96.20% <100.00%> (+0.04%)	⬆️
...nternal/xds/xdsclient/xdsresource/unmarshal_lds.go	93.41% <100.00%> (+0.03%)	⬆️
...nternal/xds/xdsclient/xdsresource/unmarshal_rds.go	87.84% <100.00%> (+0.09%)	⬆️
internal/xds/clients/xdsclient/channel.go	78.67% <62.50%> (-3.28%)	⬇️

... and 43 files with indirect coverage changes

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[arjan-bal]

Please also add a release note mentioning the bug fix.

[easwars]

I just happened to look at this PR by chance. Since @arjan-bal is already reviewing it, I'll leave it to him. I don't want both of us spending time on the PR. Thanks.

[arjan-bal]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a panic that can occur when processing an xDS ADS response with an empty resource name. The core of the fix is the addition of validation checks in the unmarshalling functions for CDS, EDS, LDS, and RDS resources to ensure resource names are not empty. A defensive check has also been added in decodeResponse to handle cases where a resource decoder might incorrectly return a nil result and nil error, preventing a potential panic. The decodeResponse function has been refactored into a method on xdsChannel for better encapsulation. All related tests have been updated to reflect these changes, including new test cases for empty resource names, ensuring the fix is well-covered. The changes are correct and improve the robustness of the xdsclient.

[arjan-bal]

LGTM