fix(react): reset missing participant attributes

[fatihcvs]

When useParticipantAttribute switches to a participant or key that does not have the requested attribute, it keeps returning the previous value. The observer's initial emission contains only attributes present on the new participant, so the changed-key filter never clears that stale state.

Reset the value from the current participant/key when the effect subscribes. Subsequent events still use the changed-key filter, and the previous subscription is cleaned up as before. Includes a React patch changeset.

Validation:

• Two regressions fail before the fix (host instead of undefined) and pass after it: switching participant and switching attribute key.
• Tests use real SDK Participant objects, updateInfo/AttributesChanged and the real hook in JSDOM. They also cover later updates, removal, ignoring old-participant events, switching back to a populated key and listener cleanup. No hook/observable mocks or server connection.
• 120 React/core/styles tests pass. React build (three package tasks), React lint (existing warnings, no errors), repository format and React API checks pass.

Developed and verified with OpenAI Codex assistance.

[changeset-bot]

🦋 Changeset detected

Latest commit: 24ec2bc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@livekit/components-react	Patch
@livekit/agents-ui	Patch
@livekit/component-example-next	Patch
@livekit/components-js-docs	Patch
@livekit/component-docs-storybook	Patch
@livekit/components-docs-gen	Patch

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