fix(core): initialize chat receivers for each topic

[fatihcvs]

Calling setupChat for two different channelTopic values in the same room only installs receivers for the first topic. The second topic gets a subject but no text/byte stream handlers or legacy data-message subscription, so its incoming messages never reach subscribers.

Check whether the topic exists in the room's topic map, instead of whether the room exists in the outer map. Repeated setup for the same topic still reuses its receivers.

Validation:

• Two regression tests fail on the original code and pass with the fix. They cover separate legacy channels, repeated setup without duplicate delivery/registration, a second text-stream channel, disconnect cleanup, and setting up both channels again.
• Tests use a real SDK Room and its events/registration methods, with an async-iterator reader test double for text chunks. They do not connect to a LiveKit server.
• All 120 core/react/styles tests pass; pnpm build:react, core lint (14 existing warnings), pnpm format:check, and React API check pass.

Includes a core patch changeset. Developed and verified with OpenAI Codex assistance.

[changeset-bot]

🦋 Changeset detected

Latest commit: 197634f

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

This PR includes changesets to release 7 packages

Name	Type
@livekit/components-core	Patch
@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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