Add apiVersion configuration support for Gemini API client

[Copilot]

Developers need to specify custom API versions (e.g., v1alpha) when using the Gemini service, matching the @google/genai library interface.

Changes

• Type definition: Added optional apiVersion?: string field to GeminiClientOptions with JSDoc
• Client implementation: Store and forward apiVersion to GoogleGenAI constructor in both _sendMessage and getEmbedding methods
• Tests: Added test coverage for clients with and without apiVersion specified

Usage

const client = new GeminiClient({
  apiKey: 'your-api-key',
  baseUrl: 'https://api.example.com',
  features: ['chat'],
  historyManager: DefaultHistoryManager,
  apiVersion: 'v1alpha'  // Optional - uses SDK default if unset
});

Backward compatible - existing code without apiVersion continues to use SDK defaults.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Support specifying apiVersion in Gemini API config</issue_title>
<issue_description>Add support for configuring the Gemini API version via the apiVersion field in configuration options, similar to the interface used in the genai library.

Developers should be able to specify a custom API version for the Gemini service when initializing the client. If not set, the default API version should be left blank and use the SDK default.

Reference: chatgpt-plugin#822

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Example reference from genai:

export interface GoogleGenAIOptions {
  ...
  /** Optional. The API version to use. 
   * If unset, the default API version will be used. 
   */
  apiVersion?: string;
  ...
}

Acceptance Criteria:

• The apiVersion field is supported in the config for Gemini API.
• If apiVersion is unset or blank, the default SDK behavior is to be used.
• Documentation is updated to clarify usage of apiVersion for Gemini API.

See also chatgpt-plugin issue 822 for the motivating scenario.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Support specifying apiVersion in Gemini API config #72

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