Support Agent Servers on remoting (#42683)

<img width="348" height="359" alt="Screenshot 2025-11-13 at 6 53 39 PM"
src="https://github.com/user-attachments/assets/6fe75796-8ceb-4f98-9d35-005c90417fd4"
/>

Also added support for per-target env vars to Agent Server Extensions

Closes https://github.com/zed-industries/zed/issues/42291

Release Notes:

- Per-target env vars are now supported on Agent Server Extensions
- Agent Server Extensions are now available when doing SSH remoting

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Co-authored-by: Lukas Wirth <me@lukaswirth.dev>
Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

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