[pull] master from supabase:master - #1168

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Refactor / hardening

## What is the current behavior?

The `update_notebook` AI tool re-fetches the notebook right before
applying operations, but concurrent edits are last-write-wins: the model
has no way to detect that the notebook changed since it planned the
edit, so a stale diff can silently overwrite someone else's changes.

## What is the new behavior?

- `get_notebook` now returns the notebook's `updated_at` timestamp.
- `update_notebook` requires a new `expected_updated_at` input field
(the `updated_at` the model got from `get_notebook`).
- At execute time, after the existing re-fetch and before applying
operations, `update_notebook` compares the fetched `updated_at` against
`expected_updated_at` and throws a descriptive error if they don't
match, telling the model to re-read the notebook and reissue the update.
- The notebook system prompt and mock tools (used by the eval harness)
are updated to match.

## Additional context

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
  * Notebook retrieval now includes the latest update timestamp.
* Notebook edits require confirmation that the content is current before
saving.

* **Bug Fixes**
  * Prevented stale edits from overwriting newer notebook changes.
* Conflicting updates are rejected, allowing the latest content to be
fetched before retrying.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->