Fix Azure OpenAI API version for o4-mini model compatibility

[Copilot]

The Azure OpenAI client was using an older API version that doesn't support the o4-mini model deployment, causing a 400 Bad Request error:

Error: Completion error: POST https://azureopenai-ineast.openai.azure.com/openai/deployments/o4-mini/chat/completions
RESPONSE 400: 400 Bad Request ERROR CODE: BadRequest
{
  "error": {
    "code": "BadRequest",
    "message": "Model o4-mini is enabled only for api versions 2024-12-01-preview and later"
  }
}

Changes:

• Added APIVersion: "2024-12-01-preview" to ClientOptions when creating Azure OpenAI clients
• Imported github.com/Azure/azure-sdk-for-go/sdk/azcore/policy package for client configuration
• Added test to verify API version is set correctly
• Only affects Azure OpenAI clients; OpenAI clients remain unchanged

The fix is minimal and surgical, targeting only the Azure client creation path while preserving all existing functionality.

Fixes #23.

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