docs: add model mappings to BYOK provider options

[R-Taneja]

Summary

Add modelMappings documentation to the byok JSDoc in gateway-provider-options.ts.

Model mappings allow BYOK credentials to include custom deployment name mappings (e.g., for Azure deployments with non-default names). Each credential in the byok record can optionally include a modelMappings array.

Example

byok: {
  azure: [{
    apiKey: '...',
    resourceName: '...',
    modelMappings: [
      { gatewayModelSlug: 'openai/gpt-5.4-nano', customModelId: 'my-deployment' }
    ]
  }]
}