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/chat.rs

@@ -10,6 +10,7 @@ use crate::provider::WireApi;
 use crate::sse::chat::spawn_chat_stream;
 use crate::telemetry::SseTelemetry;
 use codex_client::HttpTransport;
+use codex_client::RequestCompression;
 use codex_client::RequestTelemetry;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ReasoningItemContent;
@@ -80,7 +81,13 @@ impl<T: HttpTransport, A: AuthProvider> ChatClient<T, A> {
         extra_headers: HeaderMap,
     ) -> Result<ResponseStream, ApiError> {
         self.streaming
-            .stream(self.path(), body, extra_headers, spawn_chat_stream)
+            .stream(
+                self.path(),
+                body,
+                extra_headers,
+                RequestCompression::None,
+                spawn_chat_stream,
+            )
             .await
     }
 }