fix: align RAG showcase with current architecture

[TristanKruse]

Summary

Keep the RAG showcase accurate and current:

• require and lock ArchUnitPython 1.5.0
• fix the ArchUnitPython repository link
• document the valid api-to-models dependency in the PlantUML architecture
• correct the method-count explanation and typo
• assert the exact intentional dependency, metric, and diagram violations

Validation

• uv lock --check: passed
• uv run --locked pytest --tb=short -q: 12 passed, 4 xfailed
• intentional failures verified with --runxfail: 4 expected failures
• ruff check tests/test_architecture.py: passed
• git diff --check: passed