[3/4] Add executor-backed RMCP HTTP client (#18583)

### Why
The RMCP layer needs a Streamable HTTP client that can talk either
directly over `reqwest` or through the executor HTTP runner without
duplicating MCP session logic higher in the stack. This PR adds that
client-side transport boundary so remote Streamable HTTP MCP can reuse
the same RMCP flow as the local path.

### What
- Add a shared `rmcp-client/src/streamable_http/` module with:
  - `transport_client.rs` for the local-or-remote transport enum
  - `local_client.rs` for the direct `reqwest` implementation
  - `remote_client.rs` for the executor-backed implementation
  - `common.rs` for the small shared Streamable HTTP helpers
- Teach `RmcpClient` to build Streamable HTTP transports in either local
or remote mode while keeping the existing OAuth ownership in RMCP.
- Translate remote POST, GET, and DELETE session operations into
executor `http/request` calls.
- Preserve RMCP session expiry handling and reconnect behavior for the
remote transport.
- Add remote transport coverage in
`rmcp-client/tests/streamable_http_remote.rs` and keep the shared test
support in `rmcp-client/tests/streamable_http_test_support.rs`.

### Verification
- `cargo check -p codex-rmcp-client`
- online CI

### Stack
1. #18581 protocol
2. #18582 runner
3. #18583 RMCP client
4. #18584 manager wiring and local/remote coverage

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec-server/src/environment.rs

@@ -3,7 +3,9 @@ use std::sync::Arc;
 
 use crate::ExecServerError;
 use crate::ExecServerRuntimePaths;
+use crate::HttpClient;
 use crate::client::LazyRemoteExecServerClient;
+use crate::client::http_client::ReqwestHttpClient;
 use crate::file_system::ExecutorFileSystem;
 use crate::local_file_system::LocalFileSystem;
 use crate::local_process::LocalProcess;
@@ -136,6 +138,7 @@ pub struct Environment {
     exec_server_url: Option<String>,
     exec_backend: Arc<dyn ExecBackend>,
     filesystem: Arc<dyn ExecutorFileSystem>,
+    http_client: Arc<dyn HttpClient>,
     local_runtime_paths: Option<ExecServerRuntimePaths>,
 }
 
@@ -146,6 +149,7 @@ impl Environment {
             exec_server_url: None,
             exec_backend: Arc::new(LocalProcess::default()),
             filesystem: Arc::new(LocalFileSystem::unsandboxed()),
+            http_client: Arc::new(ReqwestHttpClient),
             local_runtime_paths: None,
         }
     }
@@ -202,6 +206,7 @@ impl Environment {
             filesystem: Arc::new(LocalFileSystem::with_runtime_paths(
                 local_runtime_paths.clone(),
             )),
+            http_client: Arc::new(ReqwestHttpClient),
             local_runtime_paths: Some(local_runtime_paths),
         }
     }
@@ -216,12 +221,14 @@ impl Environment {
     ) -> Self {
         let client = LazyRemoteExecServerClient::new(exec_server_url.clone());
         let exec_backend: Arc<dyn ExecBackend> = Arc::new(RemoteProcess::new(client.clone()));
-        let filesystem: Arc<dyn ExecutorFileSystem> = Arc::new(RemoteFileSystem::new(client));
+        let filesystem: Arc<dyn ExecutorFileSystem> =
+            Arc::new(RemoteFileSystem::new(client.clone()));
 
         Self {
             exec_server_url: Some(exec_server_url),
             exec_backend,
             filesystem,
+            http_client: Arc::new(client),
             local_runtime_paths,
         }
     }
@@ -243,6 +250,10 @@ impl Environment {
         Arc::clone(&self.exec_backend)
     }
 
+    pub fn get_http_client(&self) -> Arc<dyn HttpClient> {
+        Arc::clone(&self.http_client)
+    }
+
     pub fn get_filesystem(&self) -> Arc<dyn ExecutorFileSystem> {
         Arc::clone(&self.filesystem)
     }