Add file content type support for PDF and document uploads

[bledden]

Summary

This PR fixes an issue where the SDK rejects file content items (type: 'file') with a Zod validation error before requests ever reach the OpenRouter API. Users couldn't send PDFs or other documents using the documented format.

The problem: The ChatMessageContentItem type only included text, image, audio, and video variants. When users tried to send file content as documented, the SDK's internal validation failed immediately.

The fix: Added the missing ChatMessageContentItemFile type that matches what the OpenRouter API actually accepts.

Changes

• Added ChatMessageContentItemFile schema to the OpenAPI spec
• Created new TypeScript file with types and Zod validation schemas
• Updated ChatMessageContentItem union to include the file variant
• Added E2E test that confirms PDF uploads work correctly

Testing

The fix was validated by:

1. Testing the raw API with curl to confirm type: "file" is accepted
2. Building the TypeScript project (no errors)
3. Running the new E2E test that uploads a PDF and verifies the response
4. Running all existing tests to ensure nothing broke

Example usage

const response = await client.chat.send({
  model: 'anthropic/claude-sonnet-4',
  messages: [{
    role: 'user',
    content: [
      { type: 'text', text: 'What is this document about?' },
      {
        type: 'file',
        file: {
          filename: 'document.pdf',
          fileData: 'https://example.com/document.pdf'
        }
      }
    ]
  }]
});

Fixes #83

[bledden]

Schema verification: I confirmed the file content type schema against the OpenRouter PDF documentation.

The documented format is:

{
  "type": "file",
  "file": {
    "filename": "document.pdf",
    "file_data": "https://example.com/document.pdf"
  }
}

The implementation matches this exactly. I also included an optional file_id field based on the ResponseInputFile schema in the OpenAPI spec, for potential future file reference support.