Add OpenAI and Azure OpenAI providers using official SDK

[crabhit]

Fixes #37

Summary

• Adds OpenAI and Azure OpenAI provider support using the official openai npm package
• Creates unified adapter that handles both OpenAI and Azure OpenAI
• Preserves Azure-specific quirks (array schema wrapping, max_completion_tokens)

Changes

• Added openai package dependency
• New lib/ai-providers/openai-adapter.ts with both createOpenAIAdapter() and createAzureOpenAIAdapter()
• Updated lib/ai-providers/registry.ts to register new providers
• Added test script scripts/test-azure-openai.ts

Test plan

• Build passes
• Test with Azure OpenAI credentials

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