Warn for invalid CSS at-rules

[moreaki]

Fixes #2215.

Context

#2215 reports that invalid at-rules are accepted too loosely. The original example uses @page { @footnote::before { ... } }: because ::before is a pseudo-element and @footnote is a page-area at-rule, the rule should be rejected with a warning instead of being treated as a valid @footnote rule.

A later issue comment listed more malformed examples that were silently ignored or accepted, including @import::before, page-area rules with invalid preludes, and unknown nested @page rules. Some cases were already handled by f992fd5, but these examples still needed targeted checks.

Changes

• Add an explicit list of supported nested @page area rules, including margin boxes and @footnote.
• Warn and discard syntactically invalid @import rules such as @import::before "test.css" without adding duplicate warnings for URL-resolution failures that are already reported elsewhere.
• Warn and discard unknown nested @page at-rules such as @unknown.
• Warn and discard nested @page area rules with invalid preludes, including @top-left-corner super and @footnote::before.
• Keep valid rules such as @page { @footnote { ... } } and @page { @top-left { ... } } warning-free.

Scope

This is a targeted follow-up to the previous warning work. It covers the concrete cases reported in #2215 and its comments, but it is not intended as a full audit of every malformed CSS at-rule in every possible context.

Tests

• venv/bin/python -m pytest tests/css/test_errors.py::test_warnings tests/css/test_pages.py
• venv/bin/python -m pytest tests/css
• venv/bin/python -m ruff check weasyprint/css/__init__.py tests/css/test_errors.py
• git diff --check