Backport of Improve Error Handling for Missing Credentials in AppRole and UserPass into release/1.17.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #28441 to be assessed for backporting due to the inclusion of the label backport/1.17.x.

The below text is copied from the body of the original PR.

---

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

---

Overview of commits

• 7c1a834

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅