docs: clarify Test-class convention applies even in legacy files

Author: felixweinberger

CLAUDE.md

@@ -22,7 +22,9 @@ This document contains critical information about working with this codebase. Fo
 3. Testing Requirements
    - Framework: `uv run --frozen pytest`
    - Async testing: use anyio, not asyncio
-   - Do not use `Test` prefixed classes, use functions
+   - Do not use `Test` prefixed classes — write plain top-level `test_*` functions.
+     Legacy files still contain `Test*` classes; do NOT follow that pattern for new
+     tests even when adding to such a file.
    - Coverage: test edge cases and errors
    - New features require tests
    - Bug fixes require regression tests