test(o11y): add Discovery LRO tracing error integration tests

[haphungw]

Add tracing integration tests for Discovery LRO error paths.

Stacked on top of #5891

[gemini-code-assist]

Code Review

This pull request introduces integration tests for Long-Running Operation (LRO) tracing under tests/o11y, covering both Showcase (GAPIC) and Discovery-based client pollers across success, logical error, transient error, and permanent error scenarios. Feedback on the changes includes removing redundant inner #[cfg(google_cloud_unstable_tracing)] attributes since the entire file is already gated with that configuration, and addressing a discrepancy where the LRO Wait span status remains UNSET instead of ERROR during logical errors in Discovery LROs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (737706a) to head (27e37b8).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5892   +/-   ##
=======================================
  Coverage   97.90%   97.90%           
=======================================
  Files         234      234           
  Lines       59451    59451           
=======================================
+ Hits        58204    58208    +4     
+ Misses       1247     1243    -4     

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