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

---------

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

crates/extension/src/extension_manifest.rs

@@ -193,6 +193,36 @@ pub struct TargetConfig {
     /// If not provided and the URL is a GitHub release, we'll attempt to fetch it from GitHub.
     #[serde(default)]
     pub sha256: Option<String>,
+    /// Environment variables to set when launching the agent server.
+    /// These target-specific env vars will override any env vars set at the agent level.
+    #[serde(default)]
+    pub env: HashMap<String, String>,
+}
+
+impl TargetConfig {
+    pub fn from_proto(proto: proto::ExternalExtensionAgentTarget) -> Self {
+        Self {
+            archive: proto.archive,
+            cmd: proto.cmd,
+            args: proto.args,
+            sha256: proto.sha256,
+            env: proto.env.into_iter().collect(),
+        }
+    }
+
+    pub fn to_proto(&self) -> proto::ExternalExtensionAgentTarget {
+        proto::ExternalExtensionAgentTarget {
+            archive: self.archive.clone(),
+            cmd: self.cmd.clone(),
+            args: self.args.clone(),
+            sha256: self.sha256.clone(),
+            env: self
+                .env
+                .iter()
+                .map(|(k, v)| (k.clone(), v.clone()))
+                .collect(),
+        }
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]