[fix][evaluation] fix export URL encoding, add upload retry, trajectory lower-bound, goroutine leak & long-conn target validation

[xueyizheng]

Preserve signed-URL path encoding in ProcessSignURLThe experiment CSV export download link had its whole URL unescaped, which decoded the percent-encoding of the path (e.g. [, ], /, spaces, non-ASCII). That broke the signature check and corrupted the object key, so exports whose experiment name contained such characters failed to download. The path/query encoding is now passed through byte-for-byte as issued by the signing service; readable filenames are left to the download response headers.

Retry large-field upload to object storage on transient errorsLarge record fields are offloaded to object storage; the upload could fail on transient, self-healing errors. Added a fixed-interval, bounded retry helper (backoff.RetryWithMaxTimesAndInterval) and wrapped the upload with it (2 retries, 1.5s apart), rebuilding the reader on each attempt so a consumed reader never re-uploads empty content.

Use request-start time as the trajectory extraction lower bound for async targetsAsync eval targets used the record's created-at time (stamped after the async call returns) as the trace query lower bound, which dropped early spans. It now uses the request-start time, minus a 1-minute buffer, and falls back to created-at when the start time is absent.

Fix goroutine pool leak in callEvaluators on early returnWhen callEvaluators returned early (missing evaluator conf / input build failure) it skipped pool.ExecAll, so the underlying pool's resident goroutines were never released. IPool.Release() is now exposed and idempotent (sync.Once), and is deferred right after pool creation so any early return still releases it.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...modules/evaluation/application/eval_openapi_app.go	33.33%	1 Missing and 1 partial ⚠️
...es/evaluation/domain/service/url_processor_impl.go	89.47%	1 Missing and 1 partial ⚠️

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
+ Coverage   78.12%   78.13%   +0.01%     
==========================================
  Files         670      670              
  Lines       76365    76404      +39     
==========================================
+ Hits        59658    59701      +43     
+ Misses      13266    13264       -2     
+ Partials     3441     3439       -2     

Flag	Coverage Δ
unittests	78.13% <92.30%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
backend/infra/backoff/backoff.go	86.20% <100.00%> (+1.59%)	⬆️
...uation/application/convertor/experiment/openapi.go	89.87% <100.00%> (+0.05%)	⬆️
backend/modules/evaluation/domain/entity/param.go	85.18% <ø> (ø)
...aluation/domain/service/expt_run_item_turn_impl.go	85.85% <100.00%> (+0.02%)	⬆️
...d/modules/evaluation/domain/service/target_impl.go	81.73% <100.00%> (+1.08%)	⬆️
...nd/modules/evaluation/infra/storage/record_data.go	80.00% <100.00%> (+0.32%)	⬆️
backend/pkg/lang/goroutine/pool.go	90.16% <100.00%> (+0.69%)	⬆️
...modules/evaluation/application/eval_openapi_app.go	92.23% <33.33%> (-0.11%)	⬇️
...es/evaluation/domain/service/url_processor_impl.go	92.00% <89.47%> (-8.00%)	⬇️

... and 1 file with indirect coverage changes

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