feat: sort AI sessions by last prompt time (#24440)

Previously, the sessions list sorted by `MIN(started_at)` across
interceptions, so sessions with old start times but recent activity
would sink to the bottom of the list regardless of how recently they
were used.

`ListAIBridgeSessions` now sorts by `COALESCE(MAX(prompt.created_at),
MIN(started_at)) DESC`, exposed as the non-nullable `last_active_at`
field. Sessions with prompts surface by last activity; sessions with no
prompts fall back to their start time.

The original implementation used two separate columns (`last_active_at`
as a nullable prompt timestamp and `sort_at` as the non-nullable cursor
key). This revision collapses them into a single `last_active_at` that
is always set — simplifying the SQL, the Go conversion, the API type,
and the frontend.

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

coderd/database/modelqueries.go

@@ -1040,6 +1040,7 @@ func (q *sqlQuerier) ListAuthorizedAIBridgeSessions(ctx context.Context, arg Lis
 			&i.CacheReadInputTokens,
 			&i.CacheWriteInputTokens,
 			&i.LastPrompt,
+			&i.LastActiveAt,
 		); err != nil {
 			return nil, err
 		}