[2/4] Implement executor HTTP request runner (#18582)

### Why
Remote streamable HTTP MCP needs the executor to perform ordinary HTTP
requests on the executor side. This keeps network placement aligned with
`experimental_environment = "remote"` without adding MCP-specific
executor APIs.

### What
- Add an executor-side `http/request` runner backed by `reqwest`.
- Validate request method and URL scheme, preserving the transport
boundary at plain HTTP.
- Return buffered responses for ordinary calls and emit ordered
`http/request/bodyDelta` notifications for streaming responses.
- Register the request handler in the exec-server router.
- Document the runner entrypoint, conversion helpers, body-stream
bridge, notification sender, timeout behavior, and new integration-test
helpers.
- Add exec-server integration tests with the existing websocket harness
and a local TCP HTTP peer for buffered and streamed responses, with
comments spelling out what each test proves and its
setup/exercise/assert phases.

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

### Verification
- `just fmt`
- `cargo check -p codex-exec-server -p codex-rmcp-client --tests`
- `cargo check -p codex-core --test all` compile-only
- `git diff --check`
- Online full CI is running from the `full-ci` branch, including the
remote Rust test job.

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

---------

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

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

@@ -103,7 +103,9 @@ async fn run_connection(
                                 break;
                             }
                         };
-                        if outgoing_tx.send(message).await.is_err() {
+                        if let Some(message) = message
+                            && outgoing_tx.send(message).await.is_err()
+                        {
                             break;
                         }
                     } else if outgoing_tx