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/db2sdk/db2sdk.go

@@ -1038,11 +1038,12 @@ func AIBridgeSession(row database.ListAIBridgeSessionsRow) codersdk.AIBridgeSess
 			Name:      row.UserName,
 			AvatarURL: row.UserAvatarUrl,
 		}),
-		Providers: row.Providers,
-		Models:    row.Models,
-		Metadata:  jsonOrEmptyMap(pqtype.NullRawMessage{RawMessage: row.Metadata, Valid: len(row.Metadata) > 0}),
-		StartedAt: row.StartedAt,
-		Threads:   row.Threads,
+		Providers:    row.Providers,
+		Models:       row.Models,
+		Metadata:     jsonOrEmptyMap(pqtype.NullRawMessage{RawMessage: row.Metadata, Valid: len(row.Metadata) > 0}),
+		StartedAt:    row.StartedAt,
+		Threads:      row.Threads,
+		LastActiveAt: row.LastActiveAt,
 		TokenUsageSummary: codersdk.AIBridgeSessionTokenUsageSummary{
 			InputTokens:           row.InputTokens,
 			OutputTokens:          row.OutputTokens,