[fix][backend] populate FieldData key from dataset schema in data reflow

[SharkeyChen]

Previously, buildItem and buildDatasetItem passed empty string as FieldData.Key when constructing dataset items. This caused:

• Evaluation set items to be silently dropped (due to fd.Key != "" filter)
• Dataset items to potentially mismatch schema when column names contain CJK characters or spaces

Fix by retrieving the real key from dataset/evaluation-set schema:

• trace_export_service: use dataset.GetFieldSchemaKeyByName in buildItem
• auto_evaluate processor: parse schema JSON from TaskRun config to get real keys via fillDatasetKeysFromSchema, avoiding extra RPC calls

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
+ Coverage   77.81%   77.82%   +0.01%     
==========================================
  Files         670      670              
  Lines       76100    76151      +51     
==========================================
+ Hits        59217    59268      +51     
+ Misses      13421    13419       -2     
- Partials     3462     3464       +2     

Flag	Coverage Δ
unittests	77.82% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...in/task/service/taskexe/processor/auto_evaluate.go	83.43% <100.00%> (+0.13%)	⬆️
...ity/domain/task/service/taskexe/processor/utils.go	100.00% <100.00%> (ø)
...ility/domain/trace/service/trace_export_service.go	74.29% <100.00%> (+0.07%)	⬆️

... and 13 files with indirect coverage changes

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