Treat selection hits on DOM-detached widgets as misses

[uttam12331]

Fixes #6643.

Problem

As diagnosed by @abs1u in the issue: during Markdown.update(), an old MarkdownParagraph can be detached from the DOM while the compositor still returns it from get_widget_and_offset_at(). A MouseDown during that window crashes:

File "src/textual/screen.py", line 1914, in _forward_event
    event.screen_offset - container.region.offset,
AttributeError: 'NoneType' object has no attribute 'region'

because container = content_widget.parent is None for the detached widget.

Fix

Following the direction the reporter verified locally ("ignoring a hit when widget.is_attached is false prevents the crash while preserving normal text selection"): in both mouse selection paths of Screen._forward_event (MouseDown start-selection and MouseMove update-selection), if the hit widget is no longer attached to the DOM, treat the hit as a miss.

Verification

• Ran the issue's race reproducer against main: crashes at screen.py:1914 exactly as reported. With this change: the race window is still observed (detached at step: 3) and there is no crash.
• Added test_mouse_down_on_detached_widget_does_not_crash, a deterministic regression test (removes a widget, then simulates the compositor still mapping it). It fails on main with the reported AttributeError and passes with this change.
• Full tests/test_selection.py suite passes (15 passed); black --check clean; CHANGELOG entry added.