fix(extensions): Table Browser must require PK and not discard dirty edits

[ZhuchkaTriplesix]

Summary

Extension Table Browser is fail-open vs native Table Browser after #771 / #772 / #773.

1. Staging attaches whenever supportsMutations is true — no PK / schema loaded check.
2. ExtensionDriverSession.getTableSchema catches errors and returns an empty TableSchemaMeta. Save then builds UPDATE/DELETE WHERE from every column.
3. Refresh, pagination, filter, and table switch dispose() the staging buffer with no confirm. No UnsavedWorkRegistry.
4. Success toast uses affectedRows ?? mutations.length — a driver that omits or zeroes affectedRows still looks like a full success. No expectDmlMatchedRows.

Repro

• Open an extension table with mutations enabled, edit a cell, change page / Refresh → edits gone.
• Driver without PK or with a failing db.getTableSchema → cells still editable; Save can update multiple rows.

Scope

• Disable staging without a real PK; treat schema load failure as unavailable (not empty PK).
• Confirm before discarding dirty edits (pagination / Refresh / table switch / Home).
• Fail Save when affectedRows is 0 / missing, matching native DML.

Out of scope

• Native SQL-grid Execute discard (separate issue).

[ZhuchkaTriplesix]

Resolved by #935 (already merged into dev; not auto-closed since this repo's default branch is main and PRs merge into dev).