Fix forced breaks in flex rows

[moreaki]

Summary

This PR adds a narrow forced-break path for row flex containers. When a row flex item requests a forced page break before or after another item, the flex line is split at that item boundary and the remaining items resume on the next page.

Part of #2400.

Root Cause

Flex layout already delegates final child layout to the normal block layout machinery, but it did not inspect break opportunities between flex items. As a result, break-before: page and break-after: page on row flex items were ignored, even though block and table layout already honor the same break values between sibling boxes.

Changes

• Reuse block_level_page_break and force_page_break to detect forced page breaks between row flex items.
• Split the current flex line at the forced break boundary.
• Resume layout from the first item after the forced break on the next page.
• Add coverage for break-before: page, break-after: page, and a wrapped-row boundary.

Known Remaining Scope

This intentionally handles a small, reviewable slice of flex breaks. Remaining work includes:

• break-before on the first flex item, which needs parent/container-level coordination to avoid pointless blank pages.
• Full left / right / recto / verso behavior and page-name selection tests.
• avoid values, which require finding earlier/later legal flex-line break opportunities.
• Column flex direction interactions without regressing existing column fragmentation behavior.
• break-inside: avoid for row flex items, especially when one item fragments and another should remain unbroken.
• Interactions between forced sibling breaks and already-fragmented flex items.
• Nested table/grid/flex children whose own break signals need to compose with flex-line breaks.

Validation

• venv/bin/python -m pytest tests/layout/test_flex.py
• venv/bin/python -m pytest tests/layout
• venv/bin/python -m ruff check weasyprint/layout/flex.py tests/layout/test_flex.py