feat: add temperature and other common settings to model-level configuration

[robert-j-y]

Description

Fixes #387

Adds support for setting default values for common model parameters at model creation time. Previously, settings like temperature could only be passed at call time via generateText/streamText. Now users can set defaults when creating the model:

// Before: TypeScript error - temperature not allowed
const model = openrouter('google/gemini-3-flash-preview', {
  temperature: 0 // ❌ Error
});

// After: Works correctly
const model = openrouter('google/gemini-3-flash-preview', {
  temperature: 0,
  topP: 0.9,
  maxTokens: 100,
});

Call-level options still override model-level settings when provided.

New settings added to OpenRouterSharedSettings:

• temperature - Controls randomness (0-2)
• topP - Nucleus sampling (0-1)
• topK - Top-k sampling
• frequencyPenalty - Penalize repeated tokens (-2 to 2)
• presencePenalty - Penalize tokens based on presence (-2 to 2)
• maxTokens - Maximum tokens to generate

Key areas for review:

• Nullish coalescing (??) is used so call-level undefined falls back to model settings, but explicit values always win
• Both chat and completion models implement the same fallback logic
• Regression test uses exact code pattern from issue Support temperature settings in OpenRouterChatSettings. (Missing) #387

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

---

Link to Devin run: https://app.devin.ai/sessions/ad14476ac9334b8a962f771f56e48975
Requested by: Robert Yeakel (@robert-j-y)

[seannetlife]

ah, this is gonna be awesome to have. ty!

[louisgv]

LGTM!