Add feature for optional request compression (#8767)

Adds a new feature
`enable_request_compression` that will compress using zstd requests to
the codex-backend. Currently only enabled for codex-backend so only enabled for openai providers when using chatgpt::auth even when the feature is enabled

Added a new info log line too for evaluating the compression ratio and
overhead off compressing before requesting. You can enable with
`RUST_LOG=$RUST_LOG,codex_client::transport=info`

```
2026-01-06T00:09:48.272113Z  INFO codex_client::transport: Compressed request body with zstd pre_compression_bytes=28914 post_compression_bytes=11485 compression_duration_ms=0
```

codex-rs/codex-api/src/endpoint/streaming.rs

@@ -6,6 +6,7 @@ use crate::provider::Provider;
 use crate::telemetry::SseTelemetry;
 use crate::telemetry::run_with_request_telemetry;
 use codex_client::HttpTransport;
+use codex_client::RequestCompression;
 use codex_client::RequestTelemetry;
 use codex_client::StreamResponse;
 use http::HeaderMap;
@@ -52,6 +53,7 @@ impl<T: HttpTransport, A: AuthProvider> StreamingClient<T, A> {
         path: &str,
         body: Value,
         extra_headers: HeaderMap,
+        compression: RequestCompression,
         spawner: fn(StreamResponse, Duration, Option<Arc<dyn SseTelemetry>>) -> ResponseStream,
     ) -> Result<ResponseStream, ApiError> {
         let builder = || {
@@ -62,6 +64,7 @@ impl<T: HttpTransport, A: AuthProvider> StreamingClient<T, A> {
                 http::HeaderValue::from_static("text/event-stream"),
             );
             req.body = Some(body.clone());
+            req.compression = compression;
             add_auth_headers(&self.auth, req)
         };