Improve Error Handling for Missing Credentials in AppRole and UserPass

[ltcarbonell]

Description

This PR updates the error handling for AppRole and UserPass when login credentials are missing. Currently, a 500 error is returned, but a 400 error is more accurate and aligns with the behavior of other auth engines (e.g., LDAP).

Original output:

> curl --request POST -v \
       http://127.0.0.1:8200/v1/auth/userpass/login/mitchellh | jq -r ".auth"
...
< HTTP/1.1 500 Internal Server Error
...

New output:

> curl --request POST -v \
       http://127.0.0.1:8200/v1/auth/userpass/login/mitchellh | jq -r ".auth"
...
< HTTP/1.1 400 Bad Request
...

TODO only if you're a HashiCorp employee

• Jira: https://hashicorp.atlassian.net/browse/VAULT-31002

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CI Results:
All Go tests succeeded! ✅

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[ccapurso]

Looks good!