[feat][trace] parallelize span and annotation queries in GetTrace

[cuichen01]

…n spanIDs provided

When SpanIDs are passed in the request, we already know which span IDs to query annotations for, so both queries can run concurrently via errgroup instead of sequentially waiting for spans to complete first.

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[codecov]

Codecov Report

❌ Patch coverage is 96.82540% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/modules/observability/infra/repo/trace.go	96.77%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #582      +/-   ##
==========================================
+ Coverage   78.18%   78.19%   +0.01%     
==========================================
  Files         670      670              
  Lines       76566    76608      +42     
==========================================
+ Hits        59862    59904      +42     
  Misses      13266    13266              
  Partials     3438     3438              

Flag	Coverage Δ
unittests	78.19% <96.82%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
...bservability/domain/trace/service/trace_service.go	88.59% <100.00%> (ø)
backend/modules/observability/infra/repo/trace.go	86.25% <96.77%> (+1.11%)	⬆️

... and 3 files with indirect coverage changes

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