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/common/lib.rs

@@ -12,6 +12,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use regex_lite::Regex;
 use std::path::PathBuf;
 
+pub mod context_snapshot;
 pub mod process;
 pub mod responses;
 pub mod streaming_sse;