Add container-aware Firefox tab tools

[valkenrath]

Summary

Adds container-aware Firefox tab management while preserving existing MCP tool names and text responses.

Changes

• Adds structured tab/container metadata to tab listing and open-tab responses.
• Adds activate-browser-tab and list-browser-containers.
• Allows open-browser-tab to target a Firefox container by exact name or cookieStoreId.
• Filters tab listings by container, window ID, and active tab state.

Notes

• Container resolution is exact-match only; ambiguous names require cookieStoreId.
• This PR is independent from the WebSocket readiness fix so either change can be reviewed or merged first.

Validation

• npm test in firefox-extension
• npm run build