fix(cron): ensure cron session title is never blank on failure

[allin2]

Summary

When set_session_title raises (e.g. title conflict with another session), the cron session was left with no title. The error was only logged at DEBUG level, making it invisible by default.

Motivation

Fixes #50535

When a cron job runs and the title-setting fails (DB error, title conflict), the cron session ends up with a blank title in the session sidebar/history. This degrades session recall quality and makes it hard to identify sessions.

Changes

• fix: Retry with a fallback title that includes the session's timestamp suffix (seconds) to guarantee uniqueness
• logging: Log the initial failure at WARNING level instead of DEBUG so operators can see it

Validation

The fix is defensive — it catches the exception and retries with a unique fallback title:

1. Primary title: {job_name} · {timestamp} (e.g. "Morning digest · Jun 22 08:00")
2. Fallback title: {job_name} · {session_suffix} (e.g. "Morning digest · 20260622_080045")

Checklist

• Code follows the project's style guidelines
• No unrelated changes included
• Commit message follows Conventional Commits format

[alt-glitch]

Resubmission of closed #50817 (same title/scope, same author). Competes with open #50575 and #50538, which target the same cron-title reliability issue (#50535) via different mechanisms — maintainer to choose. Related, not duplicate.

[allin2]

Closing to resubmit after verification against latest main.