[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/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 {}