Persist Cloudflare affinity cookies for MCP HTTP (#29516)

[Codex Thread
019ef1f9-36e2-7e91-9337-504f097b9dc1](https://codex-thread-link.openai.chatgpt-team.site/thread/019ef1f9-36e2-7e91-9337-504f097b9dc1)

## Why

Hosted plugin-service Streamable HTTP MCP traffic uses
`https://chatgpt.com/backend-api/ps/mcp` and depends on Cloudflare's
`__cflb` cookie for load-balancer affinity. The local and exec-server
`http/request` path built a fresh reqwest client for each request
without installing Codex's existing shared ChatGPT Cloudflare cookie
store, so affinity could be lost between calls.

This is an affinity-hardening change motivated by an incident
investigation. It does not establish the broader connector-cache
incident RCA or claim to fix that incident in full.

## What changed

- Install the existing process-local, strictly allowlisted ChatGPT
Cloudflare cookie store on the reqwest client used by
`ReqwestHttpClient`.
- Fresh clients now share allowed Cloudflare infrastructure cookies
within the process that originates the local or exec-server network
request.
- Keep the existing HTTPS ChatGPT-host and Cloudflare-cookie-name
restrictions. This does not introduce a general cookie jar or send
ChatGPT Cloudflare cookies to unrelated hosts.

## Test coverage

- `codex-client` unit coverage verifies that the existing strict store
accepts and returns `__cflb` for HTTPS ChatGPT URLs.
- The exec-server HTTPS integration test sends four independent
`http/request` calls through a local TLS-intercepting proxy and verifies
that:
- `Set-Cookie: __cflb=west` is sent on the next plugin-service request;
  - a later `Set-Cookie: __cflb=central` replaces the stored value;
  - non-Cloudflare session cookies are discarded;
  - no stored ChatGPT Cloudflare cookie is sent to a non-ChatGPT host.
- `just test -p codex-client` — 38 passed.
- `just test -p codex-exec-server --test chatgpt_cloudflare_affinity` —
1 passed.
- `just bazel-lock-check` — passed.

## Non-goals

- No persistence of ChatGPT auth, account, session, residency, or
arbitrary cookies.
- No cookie persistence for third-party MCP servers.
- No special composition of caller-provided `Cookie` headers.
- No plugin-service, connector-cache, Habitat/habicache, routing,
redirect, or API-contract changes.
- No broader incident RCA conclusions.

codex-rs/exec-server/src/client/reqwest_http_client.rs

@@ -9,6 +9,7 @@ use std::error::Error as StdError;
 use std::time::Duration;
 
 use codex_client::build_reqwest_client_with_custom_ca;
+use codex_client::with_chatgpt_cloudflare_cookie_store;
 use codex_exec_server_protocol::JSONRPCErrorError;
 use futures::FutureExt;
 use futures::StreamExt;
@@ -66,7 +67,7 @@ impl ReqwestHttpClient {
             HttpRedirectPolicy::Follow => builder,
             HttpRedirectPolicy::Stop => builder.redirect(reqwest::redirect::Policy::none()),
         };
-        build_reqwest_client_with_custom_ca(builder)
+        build_reqwest_client_with_custom_ca(with_chatgpt_cloudflare_cookie_store(builder))
             .map_err(|error| ExecServerError::HttpRequest(error.to_string()))
     }
 }