feat(setup): validation for url/base_url

[Breee]

Summary

Fixes #519 by adding stronger validation and normalization for Keycloak url / admin_url and base_path handling.

Changes

• Validate required url and optional admin_url as absolute URLs.
• Reject malformed URL values (including query/fragment).
• Normalize trailing slashes on URL fields.
• Validate and normalize base_path:
  • "/" is normalized to "" to prevent double-slash request paths.
  • trailing slash is trimmed for non-root paths.

Additional Updates

• Added unit tests covering URL/base path validation and normalization.
• Updated docs and examples with guidance for valid url/base_path values.