fix(dropdown): preserve input value when choices update via key_up

[viditkbhatnagar]

The value-sync effect in Dropdown.svelte reset input_text whenever choices changed, even during active typing. When key_up triggered a backend update with new choices, the effect saw value=null and cleared the input, also causing spurious change events.

Added is_focused guard so the value-sync effect skips resetting input_text while the user is typing. It still refreshes filtered_indices and selected_index for the new choices.

Added 3 tests covering search-as-you-type: preserving input text, no spurious change events, and showing updated options.

All 380 tests pass.

Fixes #13127

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🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/dropdown	patch
gradio	patch

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• fix(dropdown): preserve input value when choices update via key_up

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