Add Anthropic caching support to OpenRouter LLM implementation

[ferenci84]

Description

Resolves #7479

What Happens Now

When you use OpenRouter with Anthropic models (like anthropic/claude-sonnet-4, anthropic/claude-opus-4, etc.) and caching is enabled:

1. System Message Caching: If cacheSystemMessage is true, the system message will be sent with cache_control: { type: "ephemeral" }

2. Conversation Caching: If cacheConversation is true, the last two user messages in the conversation will be cached

3. Automatic Processing: OpenRouter automatically handles the caching headers internally - no additional headers needed

Example Request Body

When sending a request to OpenRouter with an Anthropic model, the modified body will look like:

{
  "model": "anthropic/claude-sonnet-4",
  "messages": [
    {
      "role": "system",
      "content": [
        {
          "type": "text",
          "text": "You are a helpful assistant",
          "cache_control": { "type": "ephemeral" }
        }
      ]
    },
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "Previous context...",
          "cache_control": { "type": "ephemeral" }
        }
      ]
    },
    {
      "role": "assistant",
      "content": "Response..."
    },
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "Current question",
          "cache_control": { "type": "ephemeral" }
        }
      ]
    }
  ]
}

Non-Anthropic Models

When using non-Anthropic models (like GPT-4, Llama, etc.) through OpenRouter, the caching fields are NOT added, ensuring compatibility with all model providers.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Tests

OpenRouter.vitest.ts is added.

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Summary by cubic

Add Anthropic caching support to OpenRouter for Claude models. When caching is enabled, we annotate the system and last two user messages with cache_control so OpenRouter handles caching; other models are unchanged.

• New Features
  • Detect Anthropic/Claude models and modify the chat body when cacheBehavior is set.
  • Add cache_control: ephemeral to the system message (cacheSystemMessage) and the last two user messages (cacheConversation); for array content, tag the last text part.
  • Leave non-Anthropic models untouched; added vitest coverage for these cases.

[RomneyDa]

@ferenci84 this will be awesome! some nitpicks, the completion option is the only important thing

[RomneyDa]

This is redundant, since if includes anthropic/claude will always include claude

[RomneyDa]

Also handle the deprecated but relevant completion option: completionOptions.promptCaching
See relevant hotfix here: #7652