Fix Document enumeration for Dictionary types to resolve WebAuthn serialization errors

[AlexDaines]

Description

Fixed a serialization bug in the Document class where the IEnumerator IEnumerable.GetEnumerator() method always attempted to convert Documents to Lists, causing InvalidDocumentTypeConversionException when the Document contained Dictionary structures. Updated the method to handle both Dictionary and List types, matching the pattern already used in the non-generic IEnumerable.GetEnumerator() method.

Motivation and Context

This change fixes a critical issue where WebAuthn operations like StartWebAuthnRegistrationAsync were failing with serialization errors. The WebAuthn service returns response data as Dictionary structures, but the Document enumeration logic was incorrectly assuming all Documents were Lists during JSON serialization.
Fixes #3837

Testing

• Verified that the fix resolves the WebAuthn StartWebAuthnRegistrationAsync serialization error
• Confirmed the implementation follows the existing pattern used in the same class for the non-generic IEnumerable.GetEnumerator() method
• Tested that Dictionary-type Documents can now be properly enumerated during JSON serialization

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license