Request compression.

Add new model_provider flag for compression to enable request
compression.  We support zstd and gzip, server also supports brotli

You can test this against the sign in with chatgpt flow by adding the
following profile:

```
[profiles.compressed]
name = "compressed"
model_provider = "openai-zstd"

[model_providers.openai-zstd]
name = "OpenAI (ChatGPT, zstd)"
wire_api = "responses"
request_compression = "zstd"
requires_openai_auth = true
```

This will zstd compress your request before sending it to the server.

codex-rs/core/tests/common/lib.rs

@@ -11,11 +11,15 @@ use regex_lite::Regex;
 use std::path::PathBuf;
 
 pub mod process;
+pub mod request;
 pub mod responses;
 pub mod streaming_sse;
 pub mod test_codex;
 pub mod test_codex_exec;
 
+pub use request::RequestBodyExt;
+pub use request::body_contains;
+
 #[track_caller]
 pub fn assert_regex_match<'s>(pattern: &str, actual: &'s str) -> regex_lite::Captures<'s> {
     let regex = Regex::new(pattern).unwrap_or_else(|err| {