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>

docs/reference/api/aibridge.md

@@ -214,6 +214,7 @@ curl -X GET http://coder-server:8080/api/v2/aibridge/sessions \
         "name": "string",
         "username": "string"
       },
+      "last_active_at": "2019-08-24T14:15:22Z",
       "last_prompt": "string",
       "metadata": {
         "property1": null,