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/codex-client/src/chatgpt_cloudflare_cookies.rs

@@ -128,11 +128,12 @@ mod tests {
     fn stores_and_returns_cloudflare_cookies_for_chatgpt_hosts() {
         let store = ChatGptCloudflareCookieStore::default();
         let url = reqwest::Url::parse("https://chatgpt.com/backend-api/codex/responses").unwrap();
+        let load_balancer = HeaderValue::from_static("__cflb=west; Path=/; Secure; HttpOnly");
         let cfuvid = HeaderValue::from_static("_cfuvid=visitor; Path=/; Secure; HttpOnly");
         let clearance =
             HeaderValue::from_static("cf_clearance=clearance; Path=/; Secure; HttpOnly");
 
-        store.set_cookies(&mut [&cfuvid, &clearance].into_iter(), &url);
+        store.set_cookies(&mut [&load_balancer, &cfuvid, &clearance].into_iter(), &url);
 
         let mut cookies = store
             .cookies(&url)
@@ -148,6 +149,7 @@ mod tests {
         assert_eq!(
             cookies,
             vec![
+                "__cflb=west".to_string(),
                 "_cfuvid=visitor".to_string(),
                 "cf_clearance=clearance".to_string()
             ]