🎯 Changes

@tanstack/ai-openrouter's text adapter collapsed systemPrompts to a plain joined string and dropped the object-form metadata, so Anthropic-family prompt caching over OpenRouter was unreachable — a cache_control breakpoint never reached the wire. (OpenAI routes auto-cache prefixes, so only Anthropic-family routes were affected.)

This mirrors what @tanstack/ai-anthropic already does:

• Declare OpenRouterSystemPromptMetadata ({ cache_control?: ChatContentCacheControl }) and thread it through BaseTextAdapter's TSystemPromptMetadata param, so cache_control is typed and autocompleted at the chat() call site.
• In mapOptionsToRequest, when any system prompt carries cache_control, emit the system message as a ChatContentText[] content array with cacheControl on the part(s) that declared it. Without cache_control, the system message is sent as the same joined string as before — existing callers are unaffected.

chat({
  adapter: openRouterText('anthropic/claude-sonnet-4.5'),
  systemPrompts: [
    {
      content: 'Large, stable instructions — cache me.',
      metadata: { cache_control: { type: 'ephemeral' } },
    },
    'Volatile per-request instruction.',
  ],
})

Tests cover: a single cached prompt (asserting both the camelCase pre-serialization shape and the snake_case cache_control wire output via ChatRequest$outboundSchema), mixed cached/uncached prompts (breakpoint only on the prompt that declared it), and the unchanged no-cache_control path (foreign metadata still dropped, plain string preserved).

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• New Features
  • System prompts can now include an optional per-prompt cache_control directive, which is forwarded to OpenRouter on supported routes.
  • Added a new public metadata type to correctly describe systemPrompts entries that carry cache control.
• Bug Fixes
  • Fixed system-prompt caching for Anthropic-family models by preserving and forwarding per-prompt cache directives instead of dropping them.
• Tests
  • Updated and expanded test coverage to validate correct serialization and that only the relevant prompt includes cache_control.