[prompts] Add <environment_context> (#1869)

## Summary
Includes a new user message in the api payload which provides useful
environment context for the model, so it knows about things like the
current working directory and the sandbox.

## Testing
Updated unit tests

codex-rs/core/src/client.rs

@@ -34,7 +34,6 @@ use crate::error::Result;
 use crate::flags::CODEX_RS_SSE_FIXTURE;
 use crate::model_provider_info::ModelProviderInfo;
 use crate::model_provider_info::WireApi;
-use crate::models::ContentItem;
 use crate::models::ResponseItem;
 use crate::openai_tools::create_tools_json_for_responses_api;
 use crate::protocol::TokenUsage;
@@ -146,15 +145,7 @@ impl ModelClient {
             vec![]
         };
 
-        let mut input_with_instructions = Vec::with_capacity(prompt.input.len() + 1);
-        if let Some(ui) = prompt.get_formatted_user_instructions() {
-            input_with_instructions.push(ResponseItem::Message {
-                id: None,
-                role: "user".to_string(),
-                content: vec![ContentItem::InputText { text: ui }],
-            });
-        }
-        input_with_instructions.extend(prompt.input.clone());
+        let input_with_instructions = prompt.get_formatted_input();
 
         let payload = ResponsesApiRequest {
             model: &self.config.model,