[flora131]

Closes #3807

Summary

origin/main already fixes the reported behavior: commit 1edfc9255 replaced the bare AggregateError with ExtensionStartupError, whose message is Extension startup failed: <extension path>: <cause> and which has an enumerable failures list. This PR adds the missing regression coverage for the issue's scenario and attributes the existing changelog entry to #3807. No source behavior changes.

Changes

• packages/coding-agent/test/extension-startup-failure-message.test.ts (new): a throwing session_start handler (cause Cannot read properties of undefined (reading 'fg')) is named in error.message, String(error), the JSON of failures, and errors; a second test checks two failing extensions in load order.
• test/unit/workflow-stage-extension-startup-fallback.test.ts: (#3807) test checking the run error, stage error/failureMessage, and serialized journal entries.
• packages/coding-agent/CHANGELOG.md: added the "Extension startup failed" hides the failing extension and cause — AggregateError members dropped from every surface #3807 link to the existing Unreleased entry.

Not covered

• The orchestrating session's log (the issue's fourth surface) is not tested.
• Other bare AggregateErrors ("Extension shutdown failed", "Session event callbacks failed") are out of scope. The theme.fg crash is Interactive workflow stages bind ctx.ui.theme as undefined, crashing UI-reading extensions at session_start #3801.

Verification

Assistant-workflow: issues-to-merged-prs (run 7286d4f0-78e1-431b-a798-87da91fc30fc)
Assistant-verification: regression fails before passed: reverting to the bare message fails the new tests (4 unit, 2 coding-agent)
Assistant-verification: vitest unit passed: 45 tests
Assistant-verification: vitest coding-agent passed: 184 tests
Assistant-verification: npm run check passed: biome, bun typecheck, shrinkwrap
Assistant-duration: 58m converged, estimated 2h
Assistant-verification: GitHub Actions CI passed: test (all platforms) on b023b67 (unit, integration, agent-suite, release-archive on linux-x64 and windows-x64; static-checks; CodeQL)
Assistant-verification: Greptile review passed: approved b023b67 with confidence 5/5 and no new comments
Co-authored-by: Norin Lavaee nlavaee@umich.edu

User-preference: loop each PR until CI is green and Greptile has no feedback, then merge

Confidence Score: 5/5

No blocking issues identified in these test and changelog changes; the reviewed changes are safe to merge.

Summary

Adds regression tests for extension startup errors and links the existing changelog entry to #3807. No production behavior changes.

• Tests pin extension names and causes in session and workflow failure reports.

_{Reviews (3) · Last reviewed commit: "test(coding-agent): type startup failure..." · Reviewed by Greptile}