add Azure OpenAI LLM provider

[yai-dev]

Summary

Add an Azure OpenAI provider to @agentrail/core.

Background

Many enterprise deployments access OpenAI models through Azure OpenAI Service rather than the public OpenAI API. A native provider gives Agentrail a cleaner configuration path for that environment.

Proposed Implementation

The current repo already depends on the openai package, which includes Azure client support.

1. Implement azure-openai-llm-provider.ts in packages/core/src/llm/providers/
2. Add side-effect registration export(s) through the current providers index
3. Reuse shared message-mapping logic where practical
4. Add tests in the current core provider test setup

Acceptance Criteria

• provider: "azure-openai" works in ModelConfig
• Text streaming works
• Tool calling works
• baseUrl / endpoint mapping is documented clearly
• Tests pass
• Changeset included