fix(credential-password): allow password min length below default 8

[sayem314]

Refs #30

Initialize compared the configured password min length against
defaultMinPasswordLength (8) while the error message claimed a floor of 4,
so valid lower values like WithPasswordMinLength(6) were rejected.

• Add minAllowedPasswordLength (4) as the true absolute floor, distinct
  from the default of 8
• Validate the configured value against that floor, accepting length >= 4
• Format the error message with the actual constant instead of a
  hardcoded "4"

Default behavior (8) is unchanged.