core: snapshot tests for compaction requests, post-compaction layout, some additional compaction tests (#11487)

This PR keeps compaction context-layout test coverage separate from
runtime compaction behavior changes, so runtime logic review can stay
focused.

## Included
- Adds reusable context snapshot helpers in
`core/tests/common/context_snapshot.rs` for rendering model-visible
request/history shapes.
- Standardizes helper naming for readability:
  - `format_request_input_snapshot`
  - `format_response_items_snapshot`
  - `format_labeled_requests_snapshot`
  - `format_labeled_items_snapshot`
- Expands snapshot coverage for both local and remote compaction flows:
  - pre-turn auto-compaction
  - pre-turn failure/context-window-exceeded paths
  - mid-turn continuation compaction
  - manual `/compact` with and without prior user turns
- Captures both sides where relevant:
  - compaction request shape
  - post-compaction history layout shape
- Adds/uses shared request-inspection helpers so assertions target
structured request content instead of ad-hoc JSON string parsing.
- Aligns snapshots/assertions to current behavior and leaves explicit
`TODO(ccunningham)` notes where behavior is known and intentionally
deferred.

## Not Included
- No runtime compaction logic changes.
- No model-visible context/state behavior changes.

codex-rs/core/tests/suite/model_switching.rs

@@ -29,7 +29,6 @@ use core_test_support::skip_if_no_network;
 use core_test_support::test_codex::test_codex;
 use core_test_support::wait_for_event;
 use pretty_assertions::assert_eq;
-use serde_json::Value;
 use wiremock::MockServer;
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
@@ -324,32 +323,14 @@ async fn model_change_from_image_to_text_strips_prior_image_content() -> Result<
     assert_eq!(requests.len(), 2, "expected two model requests");
 
     let first_request = requests.first().expect("expected first request");
-    let first_has_input_image = first_request.inputs_of_type("message").iter().any(|item| {
-        item.get("content")
-            .and_then(Value::as_array)
-            .is_some_and(|content| {
-                content
-                    .iter()
-                    .any(|span| span.get("type").and_then(Value::as_str) == Some("input_image"))
-            })
-    });
     assert!(
-        first_has_input_image,
+        !first_request.message_input_image_urls("user").is_empty(),
         "first request should include the uploaded image"
     );
 
     let second_request = requests.last().expect("expected second request");
-    let second_has_input_image = second_request.inputs_of_type("message").iter().any(|item| {
-        item.get("content")
-            .and_then(Value::as_array)
-            .is_some_and(|content| {
-                content
-                    .iter()
-                    .any(|span| span.get("type").and_then(Value::as_str) == Some("input_image"))
-            })
-    });
     assert!(
-        !second_has_input_image,
+        second_request.message_input_image_urls("user").is_empty(),
         "second request should strip unsupported image content"
     );
     let second_user_texts = second_request.message_input_texts("user");