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.
