remote: Reuse existing SSH ControlMaster sessions

[nicosalm]

Closes #45271

Zed hardcodes ControlMaster=yes with a ControlPath in a random temp directory, so it can never find a ControlMaster session the user already has open. This means you get prompted for credentials again even if you're already authenticated.

This patch checks for an existing master before spawning a new one. It runs ssh -G to resolve the user's configured controlpath (with %h, %p, etc. already expanded), then ssh -O check to verify it's alive. If it is, Zed skips askpass and uses the existing socket. If not, the current behavior is unchanged. Both commands are local and don't hit the network.

Also adds a killed: AtomicBool to SshRemoteConnection because has_been_killed() was using master_process.is_none() as a proxy for "connection is dead." When reusing an external master, master_process starts as None, so the connection pool would discard it immediately.

Tested against a university SSH setup with ControlMaster auto configured. The reuse path connects without prompting, and the fallback path works normally when no master exists. Windows is unaffected.

Release Notes:

• Zed now reuses existing SSH ControlMaster sessions instead of prompting for credentials again (Existing SSH Sessions Not Reused with ControlMaster #45271).

[cla-bot]

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[nicosalm]

@cla-bot check

[cla-bot]

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[Veykril]

Thanks, and sorry for the late review!

[yeskunall]

Hey @nicosalm -- improvements like this make the DX better for everyone. Thanks for working on this and congratulations on your first contribution to Zed! 💖