[prompt] xml-format EnvironmentContext (#2272)

## Summary
Before we land #2243, let's start printing environment_context in our
preferred format. This struct will evolve over time with new
information, xml gives us a balance of human readable without too much
parsing, llm readable, and extensible.

Also moves us over to an Option-based struct, so we can easily provide
diffs to the model.

## Testing
- [x] Updated tests to reflect new format

codex-rs/core/tests/prompt_caching.rs

@@ -89,10 +89,15 @@ async fn prefixes_context_and_instructions_once_and_consistently_across_requests
     let shell = default_user_shell().await;
 
     let expected_env_text = format!(
-        "<environment_context>\nCurrent working directory: {}\nApproval policy: on-request\nSandbox mode: read-only\nNetwork access: restricted\n{}</environment_context>",
+        r#"<environment_context>
+  <cwd>{}</cwd>
+  <approval_policy>on-request</approval_policy>
+  <sandbox_mode>read-only</sandbox_mode>
+  <network_access>restricted</network_access>
+{}</environment_context>"#,
         cwd.path().to_string_lossy(),
         match shell.name() {
-            Some(name) => format!("Shell: {name}\n"),
+            Some(name) => format!("  <shell>{}</shell>\n", name),
             None => String::new(),
         }
     );
@@ -190,12 +195,10 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
         .unwrap();
     wait_for_event(&codex, |ev| matches!(ev, EventMsg::TaskComplete(_))).await;
 
-    // Change everything about the turn context.
-    let new_cwd = TempDir::new().unwrap();
     let writable = TempDir::new().unwrap();
     codex
         .submit(Op::OverrideTurnContext {
-            cwd: Some(new_cwd.path().to_path_buf()),
+            cwd: None,
             approval_policy: Some(AskForApproval::Never),
             sandbox_policy: Some(SandboxPolicy::WorkspaceWrite {
                 writable_roots: vec![writable.path().to_path_buf()],
@@ -227,7 +230,6 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
 
     let body1 = requests[0].body_json::<serde_json::Value>().unwrap();
     let body2 = requests[1].body_json::<serde_json::Value>().unwrap();
-
     // prompt_cache_key should remain constant across overrides
     assert_eq!(
         body1["prompt_cache_key"], body2["prompt_cache_key"],
@@ -243,16 +245,13 @@ async fn overrides_turn_context_but_keeps_cached_prefix_and_key_constant() {
         "content": [ { "type": "input_text", "text": "hello 2" } ]
     });
     // After overriding the turn context, the environment context should be emitted again
-    // reflecting the new cwd, approval policy and sandbox settings.
-    let shell = default_user_shell().await;
-    let expected_env_text_2 = format!(
-        "<environment_context>\nCurrent working directory: {}\nApproval policy: never\nSandbox mode: workspace-write\nNetwork access: enabled\n{}</environment_context>",
-        new_cwd.path().to_string_lossy(),
-        match shell.name() {
-            Some(name) => format!("Shell: {name}\n"),
-            None => String::new(),
-        }
-    );
+    // reflecting the new approval policy and sandbox settings. Omit cwd because it did
+    // not change.
+    let expected_env_text_2 = r#"<environment_context>
+  <approval_policy>never</approval_policy>
+  <sandbox_mode>workspace-write</sandbox_mode>
+  <network_access>enabled</network_access>
+</environment_context>"#;
     let expected_env_msg_2 = serde_json::json!({
         "type": "message",
         "id": serde_json::Value::Null,