fix: handle frontend error events without an Error object

[flcrom]

Closes #7482.

window.onerror read error.name and error.message unconditionally. Some error events have no Error object, such as the benign ResizeObserver loop notification, so the handler threw and nothing reached handle_frontend_exception.

When there is no Error object, the handler now reports the message plus url:line:column instead. The existing path is unchanged when an Error is present.

Added a test to state_js.test.mjs for the no-Error case (the existing setup gained a react override so the hook can be exercised). node --experimental-vm-modules --test tests/units/compiler/state_js.test.mjs passes locally, 19/19. I did not run the full test suite or lint locally; CI will.

News fragment included.

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Type of change

• Bug fix (non-breaking change which fixes an issue)

[cubic-dev-ai]

No issues found across 3 files

_{View guided diff | Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge, with the previous test-coverage concern fully addressed.

Summary

window.onerror now reports the message and source location when an error event has no Error object.

• Frontend errors without an Error object still reach the backend.

_{Reviews (2) · Last reviewed commit: "test: assert exact payload for the no-Er..." · Reviewed by Greptile}

[codspeed]

Merging this PR will not alter performance

⚠️ 14 benchmarks spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

• test_import_cli
• test_cli_startup[version]
• test_cli_startup[help]
• test_cli_startup[deploy_help]
• test_state_manager_memory_read_only_modify
• test_get_state[memory]
• test_emit_update
• test_modify_state[disk]
• test_set_state[disk]
• test_state_manager_memory_warm_get
• …and 4 more

✅ 148 untouched benchmarks

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_{Comparing flcrom:fix-onerror-without-error-object (b42edfd) with main (2eb1b44)}

[masenf]

thanks for the patch