fix(plugin-meetings): prevent LLM connection from being permanently killed

[mickelr]

Problem

updateLLMConnection() disconnects the shared LLM singleton (code 3050, permanent) before validating whether reconnection is feasible. This causes permanent LLM loss in two scenarios:

1. Unjoined meeting triggers disconnect - An incoming CALL creates a new meeting object that calls updateLLMConnection(). Since isJoined() is false on the new meeting, it disconnects the webinar LLM but never reconnects.

2. datachannelUrl cleared by locus.difference - A locus delta clears datachannelUrl. The method disconnects the LLM, then registerAndConnect() silently bails out because datachannelUrl is null.

Fix

Add a guard before cleanupLLMConneciton() that validates reconnection is feasible (isJoined, url, and dataChannelUrl must all be truthy). If reconnection is not possible, the existing LLM connection is preserved.

Testing

• Updated existing test for the new behavior
• Added 3 new unit tests covering both failure scenarios
• All 3273 unit tests pass