fix(opencode): add fallback models for session title generation

[BenSharir]

Issue for this PR

Closes #29734

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When ensureTitle fails (rate limit, model not found, proxy error), the error was silently swallowed via Effect.ignore. This made title generation failures impossible to diagnose.

Changes:

1. Extract generateTitle helper (reused for primary + fallbacks)
2. On primary failure, iterate through all configured providers' small models
3. Replace Effect.ignore with Effect.catchCause + warning log

The fallback works because provider.list() returns all configured providers, and getSmallModel() returns each provider's best small model. If one provider's model is down/exhausted, another provider's model can generate the title.

How did you verify your code works?

• bun typecheck passes (0 errors in prompt.ts)
• Code review of Effect patterns matches existing codebase style

Screenshots / recordings

N/A - no UI changes

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found one potentially related PR:

PR #27939: feat(session): add configurable fallback model chain
#27939

This PR appears related because it also addresses fallback model chains for session functionality, which is similar in scope to PR #29735's focus on adding fallback models for session title generation. You may want to review this PR to ensure there's no overlap in approach or existing implementation.

The other PRs found (Kiro provider, per-model temperature) are less directly related to the fallback mechanism for session title generation.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[BenSharir]

Note on PR #27939: That PR adds a more comprehensive configurable fallback chain for all LLM calls (including title generation). If #27939 is merged first, this PR may become redundant or could be simplified to just the logging change (replacing Effect.ignore with warning logs).

This PR is a simpler, targeted fix that:

• Only affects title generation
• Automatically tries all providers' small models (no config needed)
• Adds warning logs for failures

Happy to close this if maintainers prefer to wait for #27939, or keep it as an interim fix.

[BenSharir]

Correction on PR #27939 overlap: After reviewing the diff, #27939 does NOT fully address this issue:

1. feat(session): add configurable fallback model chain #27939 requires user config - title fallbacks only work if user sets agent.title.fallbacks explicitly
2. feat(session): add configurable fallback model chain #27939 keeps Effect.ignore - the call site still silently swallows failures
3. No logging added - failures remain invisible

This PR (#29735) provides:

1. Automatic fallback - tries all providers' small models without config
2. Visibility - replaces Effect.ignore with warning logs

The two PRs are complementary rather than redundant. This one could land independently or be adapted to work with #27939's infrastructure.

[BenSharir]

Closing in favor of #29737 (minimal logging-only fix). The automatic fallback logic from this PR would overlap with #27939's configurable fallback chain - better to let that PR handle fallback infrastructure while this logging fix can land independently.