Include preceding assistant context in Guardian sender reviews (#49951)

## Why

Short sender replies such as `👍` need preceding assistant context to be understood during a delegation review. User messages alone omit that context.

## What changed

- Capture preceding assistant context alongside up to three recent sender user messages at delivery, for Guardian only. Label assistant messages as untrusted context rather than authorization or verified questions.
- Share the existing 900-byte budget per exchange, prioritize user evidence, and report unavailable assistant context.
- Recognize `cloud_threads.send_message` deliveries and accept both compact and indented delegation envelopes.

## Testing

Expand integration coverage for app, TUI, and cloud-thread deliveries, contextual replies, and budget-driven omissions. Assert that sender user and assistant context stays out of the delegated agent's prompt.

GitOrigin-RevId: 4eca369cbb30241635273f729dec81b78ea57faf