Extract shared HTTP transport into codex-http-client (#31323)

## Why

Codex-owned HTTP construction currently lives in `codex-client`
alongside higher-level retry, SSE, and request-telemetry policy. That
makes it difficult to apply shared network behavior consistently across
crates, particularly system proxy/PAC resolution, custom CA handling,
and the ChatGPT Cloudflare cookie policy. It also leaves no clear crate
boundary for migrating direct `reqwest` usage behind a single Codex
abstraction.

This change establishes that low-level ownership boundary without
changing request behavior. It builds on the system proxy support
introduced in #26706, #26707, #26708, and #26709.

## What changed

- Added `codex-rs/http-client` as the `codex-http-client` crate.
- Moved request/response types, the concrete `reqwest` transport, custom
CA handling, Cloudflare cookie policy, and macOS/Windows proxy
resolution into the new crate.
- Kept retry, SSE, and request-telemetry policy in `codex-client`.
- Re-exported the moved API from `codex-client`, including compatibility
aliases for `CodexHttpClient` and `CodexRequestBuilder`, so existing
consumers do not change in this PR.
- Moved the existing proxy and custom-CA tests with their
implementation.

## Scope boundary

This PR deliberately stops at the crate extraction. Stacked follow-up
#31331 migrates downstream imports from `codex-client` to
`codex-http-client`, keeping this change focused on ownership and
compatibility rather than mixing in repository-wide call-site churn.

## Review guide

GitHub reports 30 changed files, of which 17 are detected renames. A
useful review order is:

1. Review the new boundary in `codex-rs/http-client/Cargo.toml` and
`codex-rs/http-client/src/lib.rs`.
2. Review `codex-rs/codex-client/Cargo.toml` and
`codex-rs/codex-client/src/lib.rs` for what remains in the higher-level
crate and how compatibility is preserved.
3. Treat the renamed implementation and test files as moves. Their
meaningful edits are limited to crate paths and normalizing the new
crate's type names to `HttpClient` and `RequestBuilder`.
4. Review `codex-rs/Cargo.toml`, `codex-rs/Cargo.lock`, and the two
`BUILD.bazel` files as mechanical workspace integration.

## Test plan

- `just test -p codex-http-client -p codex-client` (38 tests)
- Compile-checked the unchanged `codex-api`, `codex-backend-client`,
`codex-cloud-tasks`, `codex-exec-server`, `codex-login`, and
`codex-model-provider` consumers against the compatibility re-exports.

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/31323).
* #31331
* __->__ #31323

codex-rs/http-client/src/lib.rs

@@ -0,0 +1,40 @@
+mod chatgpt_cloudflare_cookies;
+mod chatgpt_hosts;
+mod custom_ca;
+mod default_client;
+mod error;
+mod outbound_proxy;
+mod request;
+mod transport;
+
+pub use crate::chatgpt_cloudflare_cookies::with_chatgpt_cloudflare_cookie_store;
+pub use crate::chatgpt_hosts::is_allowed_chatgpt_host;
+pub use crate::custom_ca::BuildCustomCaTransportError;
+/// Test-only subprocess hook for custom CA coverage.
+///
+/// This stays public only so the `custom_ca_probe` binary target can reuse the shared helper. It
+/// is hidden from normal docs because ordinary callers should use
+/// [`build_reqwest_client_with_custom_ca`] instead.
+#[doc(hidden)]
+pub use crate::custom_ca::build_reqwest_client_for_subprocess_tests;
+pub use crate::custom_ca::build_reqwest_client_with_custom_ca;
+pub use crate::custom_ca::maybe_build_rustls_client_config_with_custom_ca;
+pub use crate::default_client::HttpClient;
+pub use crate::default_client::RequestBuilder;
+pub use crate::error::StreamError;
+pub use crate::error::TransportError;
+pub use crate::outbound_proxy::BuildRouteAwareHttpClientError;
+pub use crate::outbound_proxy::ClientRouteClass;
+pub use crate::outbound_proxy::OutboundProxyConfig;
+pub use crate::outbound_proxy::RouteFailureClass;
+pub use crate::outbound_proxy::build_reqwest_client_for_route;
+pub use crate::request::EncodedJsonBody;
+pub use crate::request::PreparedRequestBody;
+pub use crate::request::Request;
+pub use crate::request::RequestBody;
+pub use crate::request::RequestCompression;
+pub use crate::request::Response;
+pub use crate::transport::ByteStream;
+pub use crate::transport::HttpTransport;
+pub use crate::transport::ReqwestTransport;
+pub use crate::transport::StreamResponse;