[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/client_api.rs

@@ -1,5 +1,12 @@
 use std::time::Duration;
 
+use futures::future::BoxFuture;
+
+use crate::ExecServerError;
+use crate::HttpRequestParams;
+use crate::HttpRequestResponse;
+use crate::HttpResponseBodyStream;
+
 /// Connection options for any exec-server client transport.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ExecServerClientConnectOptions {
@@ -17,3 +24,22 @@ pub struct RemoteExecServerConnectArgs {
     pub initialize_timeout: Duration,
     pub resume_session_id: Option<String>,
 }
+
+/// Sends HTTP requests through a runtime-selected transport.
+///
+/// This is the HTTP capability counterpart to [`crate::ExecBackend`]. Callers
+/// use it when they need environment-owned network requests but should not
+/// depend on the concrete connection type or how that connection is established.
+pub trait HttpClient: Send + Sync {
+    /// Perform an HTTP request and buffer the response body.
+    fn http_request(
+        &self,
+        params: HttpRequestParams,
+    ) -> BoxFuture<'_, Result<HttpRequestResponse, ExecServerError>>;
+
+    /// Perform an HTTP request and return a streamed body handle.
+    fn http_request_stream(
+        &self,
+        params: HttpRequestParams,
+    ) -> BoxFuture<'_, Result<(HttpRequestResponse, HttpResponseBodyStream), ExecServerError>>;
+}