Implements sawWorkerFatal in RA InvocationEnd event.

[dom96]

Adds some observability for fatals coming out of Pyodide.

[codspeed-hq]

Merging this PR will not alter performance

✅ 70 untouched benchmarks
⏩ 129 skipped benchmarks¹

---

_{Comparing dominik/python-fatal-observability (3d8c11a) with main (8496472)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[jasnell]

Code Review (AI-generated by Claude)

Note: This review was generated by an AI assistant and may contain inaccuracies. Please use your judgment when evaluating these suggestions.

Overall this is a clean, well-scoped PR. The approach of injecting a JSG module for cross-boundary fatal error reporting follows established patterns in the pyodide integration. A few suggestions below, mostly around robustness of the fatal error path and preserving error context.

Summary of Findings

Priority	Finding	Effort
Medium	Use IoContext::tryCurrent() instead of hasCurrent() + current()	Trivial
Medium	Guard against missing IncomingRequest in getMetrics()	Small
Medium	Pass error message through the reporting stack	Medium
Low	Clarify on_fatal upstream support	Trivial
Low	Add basic test coverage	Medium

See inline comments for details.

Questions

1. Is there a corresponding Pyodide-side change that makes Module.API.on_fatal actually get called? If this hook doesn't exist in the bundled Pyodide versions (0.26.0a2 / 0.28.2), the callback registration would be inert until that lands.
2. Is sawWorkerFatal / setWorkerFatal already implemented in the internal MetricsCollector::Request? The open-source RequestObserverWithTracer in server.c++ does not override it.

[ryanking13]

Looks good to me. But probably we can wait until the meeting to discuss with the final design

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.56%. Comparing base (46f1884) to head (3d8c11a).
⚠️ Report is 58 commits behind head on main.

Files with missing lines	Patch %	Lines
src/workerd/api/pyodide/pyodide.c++	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6126      +/-   ##
==========================================
- Coverage   70.61%   70.56%   -0.06%     
==========================================
  Files         409      409              
  Lines      109295   109537     +242     
  Branches    18072    18039      -33     
==========================================
+ Hits        77179    77291     +112     
- Misses      21270    21440     +170     
+ Partials    10846    10806      -40     

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