fix(extension): keep active daemon websocket

[GreyC]

Summary

• capture each Browser Bridge WebSocket instance locally so stale close events cannot clear a newer active connection
• route hello/result/log sends through safeSend() so closing/closed sockets do not throw
• add extension coverage for a superseded WebSocket closing after a replacement connection is active

Fixes #1537.
Related to #569, #1229, and #1075. This is a small scoped service-worker WebSocket fix and does not replace the broader offscreen-document design in #569.

Verification

• npm --prefix extension run build
• npx vitest run --project extension extension/src/background.test.ts
• npm --prefix extension run typecheck