Fix slider pointer capture

[moosama76]

Objective

• Make sliders capture the pointer when dragging, so dragging continues correctly even when the pointer moves outside the slider's hit area.
• Fixes Update slider / scrollbar / number_input to use pointer capture #25705.

Solution

• Capture the pointer when a slider receives PointerDragStart.
• Release the pointer when the drag ends.
• Added a regression test verifying that the slider captures the pointer on drag start.

Testing

• Ran the focused slider pointer-capture test successfully.
• Ran the full bevy_ui_widgets test suite successfully with cargo test -p bevy_ui_widgets.
• Ran git diff --check successfully.
• No platform-specific behavior is expected from this change.

AI disclosure

I used an LLM only as a final code reviewer to improve any non rust-idiomatic code

[viridia]

Seems fine - @alice-i-cecile Would this also fall under your "low value test" criteria?

[viridia]

Also, needs an LLM disclosure in the description.

[alice-i-cecile]

This is relatively high value as a test: there's more complex logic here that could plausibly be broken.

This doesn't smell particularly AI-generated to me, but yes, AI disclosure is required.

[moosama76]

This is relatively high value as a test: there's more complex logic here that could plausibly be broken.

This doesn't smell particularly AI-generated to me, but yes, AI disclosure is required.

I added LLM disclosure