Fix initial failed-send live region crash

[OEvgeny]

Fixes #5812

Summary

• Prevent LiveRegionSendFailed from dereferencing the unset initial value returned by usePrevious().
• Treat a send failure already present at mount as an existing state, not a newly failed activity, so it is not announced.
• Add a component-level regression test for an initially failed outgoing activity.

Validation

• npm test -- --runInBand packages/component/src/Transcript/LiveRegion/SendFailed.spec.tsx
• npm run precommit

[Copilot]

Pull request overview

This PR fixes a crash in the transcript live-region “send failed” announcer that could occur on first render when usePrevious() has not yet produced a value, and adds a regression test + changelog entry to document the fix.

Changes:

• Guard LiveRegionSendFailed against the initial usePrevious() value to prevent dereferencing undefined.
• Treat an already-failed outgoing activity at mount time as pre-existing (no announcement on mount).
• Add a component-level regression test for the “failed at mount” scenario and document the fix in the changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
packages/component/src/Transcript/LiveRegion/SendFailed.tsx	Adds an initial-value guard to avoid crashing when comparing current vs previous failed-activity keys.
packages/component/src/Transcript/LiveRegion/SendFailed.spec.tsx	Adds a regression test ensuring an initially failed send status does not cause an announcement/crash.
CHANGELOG.md	Adds an Unreleased “Fixed” entry referencing issue #5812 and this PR.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.