Add tests for add_browser, list_browsers, switch_browser

[forier]

Problem

The multi-browser tools have no automated tests. Core flows should be covered to prevent regressions.

Test cases needed

add_browser

• Successfully connects to a Chrome instance on a different port
• Throws error for unreachable URL
• Throws error for duplicate browser ID
• switchTo: false adds browser without switching

list_browsers

• Returns empty list when no browsers connected (edge case)
• Returns correct active/connected status for each browser
• Shows [selected] marker on active browser

switch_browser

• Switches to a valid browser ID
• Throws clear error for unknown browser ID
• Throws clear error for disconnected browser
• Context is correctly maintained per browser after switching

Integration

• Full flow: add → switch → navigate → switch back → verify pages are independent

Notes

The original project uses Sinon for mocking. Tests should follow the same patterns.