AAI-289 better validation/error messages for username and password

[marius-mather]

Description

AAI-289: Pydantic's default error messages for string constraints are not very user-friendly, override them so we can provide custom errors that we can send to the frontend.

Changes

• Create custom types for BiocommonsUsername and BiocommonsPassword so we can set custom error messages for validation
• Tests of validation and error messages

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added unit / integration tests that prove my fix is effective or that my feature works
• I have run all tests locally and they pass
• I have updated the documentation (if applicable)

How to Test Manually (if necessary)

Run uv run pytest

[amandazhuyilan]

Very nice error messages!