Allow OpenAI apiKey to accept function for dynamic key loading

[zastrowm]

Description

The OpenAI SDK supports dynamic API key resolution through async functions, enabling use cases like credential rotation, secret manager integration, and per-request authentication. However, our SDK currently only accepts static strings for the apiKey parameter, preventing users from leveraging these capabilities.

Public API Changes

The OpenAIModelOptions.apiKey parameter now accepts either a string or an async function:

// Before: only string supported
const model = new OpenAIModel({
  modelId: 'gpt-4o',
  apiKey: 'sk-...'
})

// After: function also supported
const model = new OpenAIModel({
  modelId: 'gpt-4o',
  apiKey: async () => await secretManager.getApiKey()
})

The change is backward compatible—all existing string-based usage continues to work without modification. The function receives no arguments and must return a Promise<string>. The OpenAI SDK invokes the function before each request and validates the returned value.

Related Issues

None; noticed this when refactoring integration tests

Agent developed via zastrowm#5 and zastrowm#4

Documentation PR

N/A

Type of Change

Bug fix/New feature

Testing

How have you tested the change?

• I ran npm run check

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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