fix: restore explicit span.end() to fix span end_time regression

[zastrowm]

Description

Since v1.24.0 (PR #1293), execute_event_loop_cycle spans no longer reflect per-cycle duration. When a cycle performs tool use and recurses, the parent cycle's OTel span stays open until all recursive children complete, producing cumulative bottom-up latency instead of per-step latency in observability backends (Langfuse, Jaeger, etc.).

The root cause: event_loop_cycle() is an async generator whose body was wrapped in use_span(end_on_exit=True). Because yield keeps the context manager open across recursive cycles, all span.end() calls fire simultaneously when the generator chain unwinds.

This is a cherry-pick of the first commit from #1939 (by @Di-Is), isolated to the core span timing fix.

The fix switches to end_on_exit=False and restores explicit span.end() calls via _end_span() in end_event_loop_cycle_span() and end_model_invoke_span(), with end_span_with_error() on all exception paths.

Related Issues

Resolves #1930

Documentation PR

N/A

Type of Change

Bug fix

Testing

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Review Summary

Assessment: Approve ✅

This PR correctly fixes the OTel span timing regression by switching from end_on_exit=True to explicit span.end() calls, ensuring cycle spans end before recursion begins.

Review Notes

• Root Cause: The async generator with end_on_exit=True kept spans open across yields during recursion, causing cumulative duration instead of per-cycle duration
• Fix Approach: end_on_exit=False with explicit _end_span() calls via end_event_loop_cycle_span() and end_model_invoke_span()
• Defensive Coding: Added is_recording() check to prevent double-ending spans
• Error Handling: Good fallback chain error_message or str(error) or type(error).__name__
• Test Coverage: Comprehensive tests verify timing behavior (cycle_spans[0].end_time <= cycle_spans[1].start_time) and error paths
• Documentation PR: N/A is appropriate for this internal bug fix

All 112 tests pass. Clean fix that resolves the regression correctly.

[zastrowm]

Langfuse example trace: https://us.cloud.langfuse.com/project/cmct6aysb00eaad07kq618eto/traces/f20d9c2a38984044511511a63387da95?timestamp=2026-04-01T16:24:43.719Z