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/codex-api/src/requests/chat.rs

@@ -351,6 +351,7 @@ fn push_tool_call_message(messages: &mut Vec<Value>, tool_call: Value, reasoning
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::provider::RequestCompression;
     use crate::provider::RetryConfig;
     use crate::provider::WireApi;
     use codex_protocol::models::FunctionCallOutputPayload;
@@ -374,6 +375,7 @@ mod tests {
                 retry_5xx: true,
                 retry_transport: true,
             },
+            request_compression: RequestCompression::None,
             stream_idle_timeout: Duration::from_secs(1),
         }
     }

codex-rs/codex-api/src/requests/responses.rs

@@ -172,6 +172,7 @@ fn attach_item_ids(payload_json: &mut Value, original_items: &[ResponseItem]) {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::provider::RequestCompression;
     use crate::provider::RetryConfig;
     use crate::provider::WireApi;
     use codex_protocol::protocol::SubAgentSource;
@@ -193,6 +194,7 @@ mod tests {
                 retry_5xx: true,
                 retry_transport: true,
             },
+            request_compression: RequestCompression::None,
             stream_idle_timeout: Duration::from_secs(5),
         }
     }