[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/src/request_processors/environment_processor.rs

@@ -0,0 +1,24 @@
+use super::*;
+
+#[derive(Clone)]
+pub(crate) struct EnvironmentRequestProcessor {
+    environment_manager: Arc<EnvironmentManager>,
+}
+
+impl EnvironmentRequestProcessor {
+    pub(crate) fn new(environment_manager: Arc<EnvironmentManager>) -> Self {
+        Self {
+            environment_manager,
+        }
+    }
+
+    pub(crate) async fn environment_add(
+        &self,
+        params: EnvironmentAddParams,
+    ) -> Result<Option<ClientResponsePayload>, JSONRPCErrorError> {
+        self.environment_manager
+            .upsert_environment(params.environment_id, params.exec_server_url)
+            .map_err(|err| invalid_request(err.to_string()))?;
+        Ok(Some(EnvironmentAddResponse {}.into()))
+    }
+}