Fix text selection in namespace filter input

[codyrancher]

Summary

The parent container's @mousedown.prevent was blocking text selection in the filter input. Adding @mousedown.stop to the input prevents the event from bubbling up, allowing normal text selection behavior while preserving the dropdown's focus management.

Fixes #12658

Occurred changes and/or fixed issues

Users can now select text in the namespace filter input field using mouse interactions (double-click to select word, click-and-drag to select text).

Technical notes summary

This adds@mousedown.stop on the input, preventing them from bubbling up to the parent .ns-filter container where @mousedown.prevent would block text selection.

Important: This change preserves the focus behavior fix introduced in ad2c555 (PR #11762).

Areas or cases that should be tested

• Use the mouse to highlight text in the namespace filter
• Use the keyboard to highlight text in the namespace filter
• Clicking outside the dropdown still closes it
• Verify the focus behavior from Vue3: Namespace selector immediately deselects #11683 is still working (no unwanted focus when clicking to open/close)

Areas which could experience regressions

• Namespace filter dropdown toggle behavior - The dropdown should still open/close correctly when clicking the main dropdown area
• Focus management - The fix from Vue3: Namespace selector immediately deselects #11683 should remain intact; clicking the dropdown should not cause unwanted focus issues
• Keyboard navigation - Tab/arrow key navigation within the namespace filter should remain unaffected

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[rak-phillip]

LGTM - The change looks safe and is an improvement with regards to previous efforts to provide more consistent behavior in the NamespaceFilter.