Fix startup WebSocket readiness

[valkenrath]

Summary

Fix startup races where the MCP server receives a tool call before the Firefox extension WebSocket is open.

Changes

• Wait briefly for the extension WebSocket before sending extension-bound commands.
• Reject pending extension requests when the active socket disconnects.
• Retry stale CONNECTING sockets in the extension without creating duplicate sockets.
• Add focused reconnect/readiness tests.

Validation

• npm test in firefox-extension
• npm run build