Clamp anchored scroll offset for short content

[vamsikrishnap23]

Fixes #6652.

When an anchored VerticalScroll contains less content than the viewport,
the compositor computes the anchored scroll position as:

new_scroll_y = content_bottom - viewport_height

If the content is shorter than the viewport, this value becomes negative.
Because it is assigned directly using set_reactive, it bypasses the normal
scroll validation, allowing scroll_y to become negative. This causes the
anchored content to render with empty space above it instead of remaining
aligned to the top of the viewport.

This change clamps the computed anchored scroll offset to 0 before it is
applied, preventing invalid negative scroll positions while preserving the
existing anchoring behavior for content that exceeds the viewport height.

Changes

• Clamp the computed anchored scroll offset to a minimum of 0 in src/textual/_compositor.py.
• Add a regression test covering anchored VerticalScroll with content shorter than the viewport.
• Add a changelog entry for Anchored scroll container paints a sticking negative scroll offset when content is shorter than the viewport #6652.

Test plan

• Verified the new regression test fails on main without the fix:

FAILED tests/test_containers.py::test_anchored_vertical_scroll_short_content_stays_clamped

E       AssertionError: assert -21 == 0

• Verified the regression test passes after applying the fix.
• Ran the complete tests/test_containers.py suite:

6 passed

• Ran the full test suite:

3459 passed, 3 skipped, 4 xfailed, 1 xpassed
448 snapshot tests passed

• Verified formatting with:

python -m black --check src/textual/_compositor.py tests/test_containers.py

Checklist

• Updated CHANGELOG.md
• Added a regression test
• Verified the test fails before the fix and passes after it
• Ran the full test suite successfully
• Verified formatting with black --check