Split AI SDK helpers and add Desktop-compatible Beeper streaming

[batuhan]

Summary

• Split AI SDK convenience adapters into a separate @better-matrix-js/ai-sdk package so the chat adapter stays protocol-focused.
• Refactor chat streaming into a small streaming/ module with Beeper-only dynamic handling for Beeper homeservers.
• Align Beeper stream transport with Desktop expectations, including com.beeper.llm deltas, start/finish framing, and terminal stream cleanup.
• Keep the Matrix adapter thin while preserving generic fallback streaming for non-Beeper homeservers.

Testing

• Added and updated unit coverage for the Beeper stream envelope, stream framing, and AI SDK adapter helpers.
• Ran package typechecks and builds for @better-matrix-js/chat-adapter and @better-matrix-js/ai-sdk.
• Ran package test suites successfully for both packages.