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/requests/responses.rs

@@ -11,10 +11,18 @@ use codex_protocol::protocol::SessionSource;
 use http::HeaderMap;
 use serde_json::Value;
 
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
+pub enum Compression {
+    #[default]
+    None,
+    Zstd,
+}
+
 /// Assembled request body plus headers for a Responses stream request.
 pub struct ResponsesRequest {
     pub body: Value,
     pub headers: HeaderMap,
+    pub compression: Compression,
 }
 
 #[derive(Default)]
@@ -32,6 +40,7 @@ pub struct ResponsesRequestBuilder<'a> {
     session_source: Option<SessionSource>,
     store_override: Option<bool>,
     headers: HeaderMap,
+    compression: Compression,
 }
 
 impl<'a> ResponsesRequestBuilder<'a> {
@@ -94,6 +103,11 @@ impl<'a> ResponsesRequestBuilder<'a> {
         self
     }
 
+    pub fn compression(mut self, compression: Compression) -> Self {
+        self.compression = compression;
+        self
+    }
+
     pub fn build(self, provider: &Provider) -> Result<ResponsesRequest, ApiError> {
         let model = self
             .model
@@ -138,7 +152,11 @@ impl<'a> ResponsesRequestBuilder<'a> {
             insert_header(&mut headers, "x-openai-subagent", &subagent);
         }
 
-        Ok(ResponsesRequest { body, headers })
+        Ok(ResponsesRequest {
+            body,
+            headers,
+            compression: self.compression,
+        })
     }
 }