[vmelikyan]

Description

• The admin session list and session detail (GET /api/v2/ai/admin/agent/sessions, GET /api/v2/ai/admin/agent/sessions/{sessionId}) now return a title for each session: the latest conversation's title when it is named, otherwise the first user message on one line, truncated to 80 characters. This is the same fallback the user sessions list already uses.
• Before, the admin UI had only latestThreadTitle, which is usually Default thread or null, so almost every row showed "Untitled conversation".
• readUsefulThreadTitle, deriveTitleFromMessageParts and loadFirstUserMessageParts are now exported from SessionReadService and reused, not copied.
• The list loads first messages only for sessions on the current page that have no named conversation, so the extra query stays bounded by the page size.
• title is added to the AgentAdminSessionSummary OpenAPI schema as optional and nullable. The admin UI uses it in GoodRxOSS/lifecycle-ui#101.

Verifying Changes

• AdminService.test.ts covers the first-message title, a named conversation title winning over the first message, the page-only first-message load, and no extra query when the page is empty.
• jest for src/server/services/agent, openApiSpec and the admin routes (2181 tests), eslint and prettier pass.