feat(o11y): add error extraction to DiscoveryOperation and fix macro compatibility

[haphungw]

Introduce telemetry injections for Discovery LROs.

This ensures Discovery LROs can record rich error details upon completion, matching the behavior of standard LROs, and provides a cleaner interface for code generation.

Used at googleapis/librarian#6304.

[gemini-code-assist]

Code Review

This pull request introduces an error method to the DiscoveryOperation trait and adds helper macros record_discovery_polling_result! and record_polling_attributes! to handle telemetry and span attribute recording for Discovery LROs. The reviewer pointed out a macro hazard in record_discovery_polling_result! where arguments are evaluated multiple times, and suggested binding them to local references first to prevent unexpected behavior or performance issues.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.88%. Comparing base (1059b90) to head (9b46cbc).

Files with missing lines	Patch %	Lines
src/lro/src/internal/discovery.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5843      +/-   ##
==========================================
- Coverage   97.89%   97.88%   -0.01%     
==========================================
  Files         226      226              
  Lines       57713    57716       +3     
==========================================
  Hits        56496    56496              
- Misses       1217     1220       +3     

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