feat: disconnect idle existing Firefox sessions

[f3tchcodes]

Bugzilla: Bug 2066912

This change disconnects from an existing Firefox instance after 30 minutes of inactivity when using --connect-existing. The Firefox process itself stays open, and the MCP reconnects to the same instance on the next tool call. The idle timeout is reset by new tool activity and does not run while a tool call is still in progress.

I tested it manually by:

1. connecting to an existing Firefox and running a tool via inspector
2. waiting for the idle timeout and confirming geckodriver exits while Firefox stays open (you can refer to background processes for this)
3. calling another tool and confirming the MCP reconnects to the Firefox instance (it should reconnect to that same instance, but that specifically is not confirmed yet)

[juliandescottes]

Sorry about the delay for the review here, I need some more focused time to test this than for the usual PRs, but I will take a look today.

[f3tchcodes]

No worries, take your time.

[f3tchcodes]

Just a small note for manual testing: the idle timeout can be temporarily changed to 10 seconds locally from src/firefox/idle-connection.ts, so the disconnect/reconnect cycle can be tested without waiting 30 minutes.

[juliandescottes]

Thanks, I would like to see a simpler implementation. The idle-connection wrapper is a bit too complex for something which only matters in one specific scenario.

I suggest to drop this wrapper and instead create a timer/promise/counter inline in CallToolRequestSchema. Downside is that we can't easily cover that with tests, but I think the simplification makes it worth this.

[f3tchcodes]

Alright, I'll rewrite and make it a simpler.

[f3tchcodes]

I have made the requested changes!