xds: suppress the duplicate errors at the client side

[mswierq]

Fixes #8994

This change suppresses duplicate errors on the xds client side by skipping calling watchers for the duplicates. This also prevents generating new child names for the same broken EDS resource (EDS resource has no localities) that is being re-send by the control plane.

RELEASE NOTES:

• xds: handle receipt of duplicated errors at the client side by suppressing them in the same manner the duplicate updates are suppressed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.08%. Comparing base (cd25711) to head (35d57a6).
⚠️ Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9185      +/-   ##
==========================================
- Coverage   83.20%   83.08%   -0.12%     
==========================================
  Files         418      419       +1     
  Lines       33741    33867     +126     
==========================================
+ Hits        28073    28140      +67     
- Misses       4250     4290      +40     
- Partials     1418     1437      +19     

Files with missing lines	Coverage Δ
internal/xds/clients/xdsclient/authority.go	80.24% <100.00%> (-0.31%)	⬇️

... and 39 files with indirect coverage changes

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[eshitachandwani]

Why do we need to do this ? Can't we directly do state.md.ErrState = md.ErrState like earlier ?

[eshitachandwani]

Also instead of evaluating this as a variable and then checking can we instead do the assignment first(which I assume needs to be done for either case) and then directly in an if block we can evaluate the condition and call continue on true. WDYT ?

[eshitachandwani]

Can we have the test in https://github.com/grpc/grpc-go/blob/master/internal/xds/clients/xdsclient/test/lds_watchers_test.go or any other test file in the internal/xds/clients/xdsclient package instead of having it in a different xdsclient/tests package?
Or if there is a reason that we have added the test here , please let me know.

[eshitachandwani]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces duplicate error suppression for ADS resource updates in the xDS client, skipping watcher notifications when duplicate errors are received, and adds a corresponding test to verify this behavior. The reviewer identified a critical issue in authority.go where a nil pointer dereference panic could occur if md.ErrState is nil but uErr.Err is non-nil, and provided a code suggestion to safely initialize errState.