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/tests/sse_end_to_end.rs

@@ -9,6 +9,7 @@ use codex_api::Provider;
 use codex_api::ResponseEvent;
 use codex_api::ResponsesClient;
 use codex_api::WireApi;
+use codex_api::requests::responses::Compression;
 use codex_client::HttpTransport;
 use codex_client::Request;
 use codex_client::Response;
@@ -124,7 +125,11 @@ async fn responses_stream_parses_items_and_completed_end_to_end() -> Result<()>
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), NoAuth);
 
     let mut stream = client
-        .stream(serde_json::json!({"echo": true}), HeaderMap::new())
+        .stream(
+            serde_json::json!({"echo": true}),
+            HeaderMap::new(),
+            Compression::None,
+        )
         .await?;
 
     let mut events = Vec::new();
@@ -189,7 +194,11 @@ async fn responses_stream_aggregates_output_text_deltas() -> Result<()> {
     let client = ResponsesClient::new(transport, provider("openai", WireApi::Responses), NoAuth);
 
     let stream = client
-        .stream(serde_json::json!({"echo": true}), HeaderMap::new())
+        .stream(
+            serde_json::json!({"echo": true}),
+            HeaderMap::new(),
+            Compression::None,
+        )
         .await?;
 
     let mut stream = stream.aggregate();