Description

• Add top-level metadata field to CreateCallDTO schema in the OpenAPI spec
• Add top-level metadata field to CreateOutboundCallDTO schema for consistency
• Add top-level metadata field to Call response schema so metadata set on create is returned on read
• The field follows the same pattern as CreateAssistantDTO's existing metadata field: {"type": "object", "description": "..."}
• This fixes the .NET SDK (and all other auto-generated SDKs) missing metadata on CreateCallDto, which currently forces customers to extend the class manually or use AssistantOverrides.metadata as a workaround
• SDKs will need to be regenerated after this merge to pick up the new field

Linear: PRO-1773

Testing Steps

• Run fern check -- 0 errors (5 pre-existing warnings unrelated to this change)
• Verify JSON is valid and metadata field matches CreateAssistantDTO pattern
• Run the app locally using fern docs dev or navigate to preview deployment
• Ensure that the changed pages and code snippets work