Add form-urlencoded content-type support for gateway testing

[shoummu1]

📌 Summary

Adds application/x-www-form-urlencoded content-type support to the gateway testing interface, enabling proper testing of OAuth2 token endpoints.

OAuth2 token endpoints require application/x-www-form-urlencoded requests. Current implementation only supports JSON, making OAuth2 testing impossible via admin UI.

🔗 Related Issue

Closes: #1111

📐 Changes

• Backend: Add content-type detection in /admin/gateways/test endpoint
• Schema: Add content_type field to GatewayTestRequest (default: application/json)
• UI: Add content-type dropdown to gateway test modal
• JS: Dynamic label updates based on content-type selection

🧪 Testing

• ✅ Unit tests for form-urlencoded handling
• ✅ Integration tests for both content types
• ✅ Manual testing with OAuth2 endpoints

✅ Checklist

• Code formatted (make black isort pre-commit)
• Fully backward compatible - defaults to application/json