fix(session): retry title generation and fall back to the session model

[thom801]

Issue for this PR

Closes #42287, #30662

Type of change

• Bug fix

What does this PR do?

Session title generation runs once with the small model and silently fails: any error leaves the session with its default New session - ... title forever. ensureTitle only fires while the session has exactly one real user message, and the small-model LLM call dies without a fallback (the caller ignores it).

This PR makes two changes in ensureTitle:

• Fall back to the session model (small: false) when the small-model title request errors, with a warning log.
• Drop the single-user-message guard so a session that still has a default title retries title generation on a later message, instead of never retrying.

Trigger examples: a small model blocked by provider guardrails/ZDR (reported in #42287), or a reasoning model returning only a <think> block (#30662).

How did you verify your code works?

Added TestLLMServer.titleError to simulate title request failures, plus two tests in packages/opencode/test/session/prompt.test.ts:

• title generation falls back to a second attempt when the title request errors
• title generation retries on a later message while the title is still default

bun test test/session/ passes (415 pass, 0 fail); bun run typecheck and oxlint pass.

Checklist

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

[kvnloo]

Why this matters

Session title generation is one-shot on the small model; errors leave New session - … forever with no later retry (#42287 #30662).

Evidence

Tip cb48ac8 (base dev e03db9b):

• packages/opencode/src/session/prompt.ts — generateTitle; on small-model error, retry with session model (small: false) + warn
• drops the single-user-message-only gate so default titles can regenerate on later turns
• live: bun test test/session/prompt.test.ts -t "title" → 2/2 pass (fallback + later-message retry); same filter on dev has 0 title tests yet
• CI: standards / compliance / duplicates success

Confirms scope. Happy to help land as-is.

Ask (design, light) — prospective

1. Is stuck default title a one-off, or a class of silent one-shot side LLM jobs that need fallback + retry policy?
2. For never-again: shared “best-effort LLM side effect” helper (small → session model → give up), or per-call sites + checklist?