Fix test compatibility with latest Keycloak (HTTPS + response changes)

[tiborrr]

Description

Tests were failing on newer Keycloak versions due to stricter HTTPS behavior and changed error payloads.

This issue tracks:

• HTTPS-capable test startup (self-signed cert support)
• scheme-aware fixtures (http/https)
• less brittle assertions for version-dependent error responses
• CI smoke coverage for Keycloak 22.0, 23.0, and latest

Done criteria

• Smoke tests pass on 22.0, 23.0, latest
• Full test suite passes
• No regression for older supported Keycloak versions

I am working with Cursor on this issue. Will post a pull request soon.