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>

codersdk/aibridge.go

@@ -78,6 +78,7 @@ type AIBridgeSession struct {
 	Threads           int64                            `json:"threads"`
 	TokenUsageSummary AIBridgeSessionTokenUsageSummary `json:"token_usage_summary"`
 	LastPrompt        *string                          `json:"last_prompt,omitempty"`
+	LastActiveAt      time.Time                        `json:"last_active_at" format:"date-time"`
 }
 
 type AIBridgeSessionTokenUsageSummary struct {