fix(utils): decode padded URL-safe base64 values

[Hughhhhcoder]

Summary

• teach Base64DecodeConverter to fall back to padded URL-safe Base64 before trying the raw no-padding variant
• keep the existing UTF-8 guard so lookalike binary payloads still round-trip unchanged
• add regression coverage for both padded and unpadded URL-safe inputs

Testing

• go test ./internal/utils -run 'TestBase64DecodeConverter'
• go test ./internal/utils -run 'TestConvertFields_PathMode'