[feat][observability] add is_system field to FilterField and FieldMeta

[cuichen01]

Add optional bool is_system to ListSpans FilterField and GetTracesMetaInfo FieldMeta, allowing frontend to distinguish system fields and pass the is_system condition in trace query filters.

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Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ules/observability/application/convertor/filter.go	0.00%	5 Missing and 1 partial ⚠️
.../observability/application/convertor/trace/span.go	0.00%	2 Missing and 2 partials ⚠️
backend/modules/observability/application/trace.go	0.00%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #587      +/-   ##
==========================================
- Coverage   77.46%   77.44%   -0.02%     
==========================================
  Files         679      679              
  Lines       80880    80891      +11     
==========================================
- Hits        62652    62650       -2     
- Misses      14493    14502       +9     
- Partials     3735     3739       +4     

Flag	Coverage Δ
unittests	77.44% <0.00%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
...es/observability/domain/component/config/config.go	50.00% <ø> (ø)
backend/modules/observability/application/trace.go	78.56% <0.00%> (-0.07%)	⬇️
.../observability/application/convertor/trace/span.go	85.42% <0.00%> (-1.41%)	⬇️
...ules/observability/application/convertor/filter.go	39.39% <0.00%> (-3.94%)	⬇️

... and 1 file with indirect coverage changes

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