Fix async validation errors persist when unrelated fields are edited

[Subham-KRLX]

Fixes #3536

Changes:
Removed validateFields prop from username and email fields in SignupForm.jsx to ensure async validation errors persist when other fields change.

Updated validateSignup function in reduxFormUtils.js to merge existing async errors with synchronous validations to prevent overwriting error messages prematurely.

Verified that username and email validation errors no longer disappear when editing unrelated fields such as password or confirmPassword.

Tested locally with multiple validation scenarios; all errors behave as expected.
Ensured change complies with existing linting and testing standards.

Checklist:
has no linting errors (npm run lint)
has no test errors (npm run test)
is from a uniquely-named feature branch and is up to date with the develop branch
is descriptively named and links to an issue number, i.e. Fixes #123

[yugalkaushik]

@Subham-KRLX The issue is already being addressed by the original author through a pull request. Please ask to be assigned before working on any issue, as we avoid concurrent work. You can instead pick an issue labeled with the help wanted tag.

[Subham-KRLX]

Thank you for the clarification I apologize for the overlap I will make sure to request assignment before starting on any issue in the future I will look for help wanted issues to contribute to going forward.