[codex] support executor registry remote environments (#21323)

## Summary

Support registry-backed remote executors end to end so downstream
services can resolve an executor id into an exec-server URL and make
that environment available to Codex without relying on the legacy cloud
environments flow.

## What changed

- switch remote executor registration to the executor registry bootstrap
contract
- allow named remote environments to be inserted into
`EnvironmentManager` at runtime
- add the experimental app-server RPC `environment/add` so initialized
experimental clients can register those remote environments for later
`thread/start` and `turn/start` selection

## Validation

Ran focused validation locally:
- `cargo test -p codex-exec-server environment_manager_`
- `cargo test -p codex-exec-server
register_executor_posts_with_bearer_token_header`
- `cargo test -p codex-app-server-protocol`

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -808,6 +808,13 @@ client_request_definitions! {
         serialization: None,
         response: v2::MockExperimentalMethodResponse,
     },
+    #[experimental("environment/add")]
+    /// Adds or replaces a remote environment by id for later selection.
+    EnvironmentAdd => "environment/add" {
+        params: v2::EnvironmentAddParams,
+        serialization: global("environment"),
+        response: v2::EnvironmentAddResponse,
+    },
 
     McpServerOauthLogin => "mcpServer/oauth/login" {
         params: v2::McpServerOauthLoginParams,
@@ -1796,6 +1803,18 @@ mod tests {
             add_credits_nudge.serialization_scope(),
             Some(ClientRequestSerializationScope::Global("account-auth"))
         );
+
+        let environment_add = ClientRequest::EnvironmentAdd {
+            request_id: request_id(),
+            params: v2::EnvironmentAddParams {
+                environment_id: "remote-a".to_string(),
+                exec_server_url: "ws://127.0.0.1:8765".to_string(),
+            },
+        };
+        assert_eq!(
+            environment_add.serialization_scope(),
+            Some(ClientRequestSerializationScope::Global("environment"))
+        );
     }
 
     #[test]
@@ -2578,10 +2597,33 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_environment_add() -> Result<()> {
+        let request = ClientRequest::EnvironmentAdd {
+            request_id: RequestId::Integer(9),
+            params: v2::EnvironmentAddParams {
+                environment_id: "remote-a".to_string(),
+                exec_server_url: "ws://127.0.0.1:8765".to_string(),
+            },
+        };
+        assert_eq!(
+            json!({
+                "method": "environment/add",
+                "id": 9,
+                "params": {
+                    "environmentId": "remote-a",
+                    "execServerUrl": "ws://127.0.0.1:8765"
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_fs_get_metadata() -> Result<()> {
         let request = ClientRequest::FsGetMetadata {
-            request_id: RequestId::Integer(9),
+            request_id: RequestId::Integer(10),
             params: v2::FsGetMetadataParams {
                 path: absolute_path("tmp/example"),
             },
@@ -2589,7 +2631,7 @@ mod tests {
         assert_eq!(
             json!({
                 "method": "fs/getMetadata",
-                "id": 9,
+                "id": 10,
                 "params": {
                     "path": absolute_path_string("tmp/example")
                 }
@@ -2850,6 +2892,19 @@ mod tests {
         assert_eq!(reason, Some("mock/experimentalMethod"));
     }
 
+    #[test]
+    fn environment_add_is_marked_experimental() {
+        let request = ClientRequest::EnvironmentAdd {
+            request_id: RequestId::Integer(1),
+            params: v2::EnvironmentAddParams {
+                environment_id: "remote-a".to_string(),
+                exec_server_url: "ws://127.0.0.1:8765".to_string(),
+            },
+        };
+        let reason = crate::experimental_api::ExperimentalApi::experimental_reason(&request);
+        assert_eq!(reason, Some("environment/add"));
+    }
+
     #[test]
     fn command_exec_permission_profile_is_marked_experimental() {
         let request = ClientRequest::OneOffCommandExec {

codex-rs/app-server-protocol/src/protocol/v2/environment.rs

@@ -0,0 +1,17 @@
+use schemars::JsonSchema;
+use serde::Deserialize;
+use serde::Serialize;
+use ts_rs::TS;
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct EnvironmentAddParams {
+    pub environment_id: String,
+    pub exec_server_url: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct EnvironmentAddResponse {}

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -6,6 +6,7 @@ mod attestation;
 mod collaboration_mode;
 mod command_exec;
 mod config;
+mod environment;
 mod experimental_feature;
 mod feedback;
 mod fs;
@@ -31,6 +32,7 @@ pub use attestation::*;
 pub use collaboration_mode::*;
 pub use command_exec::*;
 pub use config::*;
+pub use environment::*;
 pub use experimental_feature::*;
 pub use feedback::*;
 pub use fs::*;