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/proto/proto/ai.proto

@@ -186,6 +186,27 @@ message ExternalAgentsUpdated {
     repeated string names = 2;
 }
 
+message ExternalExtensionAgentTarget {
+    string archive = 1;
+    string cmd = 2;
+    repeated string args = 3;
+    optional string sha256 = 4;
+    map<string, string> env = 5;
+}
+
+message ExternalExtensionAgent {
+    string name = 1;
+    optional string icon_path = 2;
+    string extension_id = 3;
+    map<string, ExternalExtensionAgentTarget> targets = 4;
+    map<string, string> env = 5;
+}
+
+message ExternalExtensionAgentsUpdated {
+    uint64 project_id = 1;
+    repeated ExternalExtensionAgent agents = 2;
+}
+
 message ExternalAgentLoadingStatusUpdated {
     uint64 project_id = 1;
     string name = 2;

crates/proto/proto/zed.proto

@@ -410,7 +410,6 @@ message Envelope {
         AgentServerCommand agent_server_command = 374;
 
         ExternalAgentsUpdated external_agents_updated = 375;
-
         ExternalAgentLoadingStatusUpdated external_agent_loading_status_updated = 376;
         NewExternalAgentVersionAvailable new_external_agent_version_available = 377;
 
@@ -436,7 +435,9 @@ message Envelope {
 
         OpenImageByPath open_image_by_path = 391;
         OpenImageResponse open_image_response = 392;
-        CreateImageForPeer create_image_for_peer = 393; // current max
+        CreateImageForPeer create_image_for_peer = 393;
+
+        ExternalExtensionAgentsUpdated external_extension_agents_updated = 394; // current max
     }
 
     reserved 87 to 88;

crates/proto/src/proto.rs

@@ -331,6 +331,7 @@ messages!(
     (GetAgentServerCommand, Background),
     (AgentServerCommand, Background),
     (ExternalAgentsUpdated, Background),
+    (ExternalExtensionAgentsUpdated, Background),
     (ExternalAgentLoadingStatusUpdated, Background),
     (NewExternalAgentVersionAvailable, Background),
     (RemoteStarted, Background),
@@ -681,6 +682,7 @@ entity_messages!(
     GitClone,
     GetAgentServerCommand,
     ExternalAgentsUpdated,
+    ExternalExtensionAgentsUpdated,
     ExternalAgentLoadingStatusUpdated,
     NewExternalAgentVersionAvailable,
     GitGetWorktrees,