fix: langfuse trace get the right parent span

[tianzhipeng-git]

the tracing event(called span in langfuse) was not organized rightly.
we should get the last not closed span as parent, so use OrderedDict to maintain the un closed span

[tianzhipeng-git]

fix issue #6846

[codspeed-hq]

CodSpeed Performance Report

Merging #6844 will improve performances by 48.39%

_{Comparing tianzhipeng-git:langfuse_parent_span (4952662) with main (0134485)}

Summary

⚡ 1 improvements
✅ 18 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_cancel_nonexistent_build	13.2 ms	8.9 ms	+48.39%