Fix right alignment with pre-wrap trailing spaces

[moreaki]

What changed

This adjusts line alignment so that trailing spaces preserved by white-space: pre-wrap hang for alignment purposes. The spaces remain in the line box, but text-align: right/end/center computes the alignment offset from the non-hanging content width.

This matches the CSS Text phase 2 rule linked from #2384: end-of-line preserved spaces with pre-wrap should hang rather than contribute to the visible right-alignment inset.

Why

In #2384, right-aligned white-space: pre-wrap text gets an unwanted right-side gap. The line content includes the preserved break-triggering trailing spaces, and WeasyPrint was using that full line width to calculate the alignment offset. As a result, the visible words were shifted too far left.

Scope

This is a small targeted fix, not full CSS Text Level 4 whitespace support. Existing collapsing-space callers keep their current behavior because trailing_whitespace_size still defaults to normal, nowrap, and pre-line; the new pre-wrap path is requested explicitly by text alignment.

Related investigation

I followed the #2384 comments and related links before making this patch:

• Right aligned text with white-space: pre-wrap #2384 points at the CSS Text white-space phase 2 hanging-space rule.
• Right aligned text with white-space: pre-wrap #2384 may be related to Right align in sample invoice  #751, but Right align in sample invoice  #751 is a separate case involving inline dl/dt/dd content and anonymous block construction around dt::before. This PR does not claim to fix Right align in sample invoice  #751.
• Right align in sample invoice  #751 is also mentioned from PDF/UA L elements contain Span as direct Kids with display: inline #2504 / PR Add tests for PDF/UA #2496 in the context of PDF/UA sample invoice testing. Those links did not include an existing whitespace alignment patch.
• I searched open and closed PRs for Right aligned text with white-space: pre-wrap #2384, Right align in sample invoice  #751, PDF/UA L elements contain Span as direct Kids with display: inline #2504, pre-wrap, white-space, hang spaces, and trailing-space terms, and did not find an existing upstream PR for this specific fix.

Tests

Added test_text_align_right_pre_wrap_trailing_space.

Validation run locally:

• venv/bin/python -m ruff check weasyprint/layout/inline.py weasyprint/layout/preferred.py tests/test_text.py
• venv/bin/python -m pytest tests/test_text.py tests/layout/test_inline.py tests/layout/test_preferred.py -q -> 294 passed, 3 xfailed
• venv/bin/python -m pytest tests/draw/test_text.py -q -> 42 passed, 5 xfailed
• venv/bin/python -m pytest -> 4026 passed, 40 xfailed

Fixes #2384.
Related to #751 and #2504.