agent: Surface title generation error reason in tooltip instead of generic "Retry"

[mvanhorn]

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments (N/A)
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

When title generation fails the tooltip just said "Title generation failed. Retry" with no indication of why. For common, actionable failures (Anthropic 200K context overflow, rate limits, auth errors), the user has no way to know whether to switch model, wait, or fix configuration without opening the thread and re-running.

Replaced the bool title_generation_failed on Thread with Option<SharedString> carrying the actual error message. Tooltip now reads the stored reason and surfaces it inline; the Retry action still works the same way. When the reason is too long, it's truncated to a single tooltip-friendly line.

Tests verify the option round-trips through retry, that successful generation clears it, and that a long upstream message gets truncated rather than overflowing.

Closes #57117

Release Notes:

• Fixed - Title generation failure tooltips now show the actual error reason (e.g. context window exceeded) instead of a generic "Retry" message.

AI was used for assistance.