Add insertOnSpace prop to control Space key selection in inline search

[Copilot]

Addresses feedback from PR #661 where Space key unconditionally triggers selection, preventing users from typing spaces in search queries (e.g., "Vue.js Framework").

Changes

• New prop insertOnSpace?: boolean - Controls Space key behavior in inline search mode. Defaults to true for backward compatibility.
• Conditional Space handling - Space only triggers selection when insertOnSpace={true}. When false, users can type spaces normally.
• Refactored selection logic - Extracted duplicate Enter/Tab/Space handling into handleInlineSelect() helper.

Usage

<!-- Default: Space selects active option -->
<SInputDropdown search="inline" :options="options" v-model="value" />

<!-- Allow typing spaces in search -->
<SInputDropdown 
  search="inline" 
  :insert-on-space="false"
  :options="options" 
  v-model="value" 
/>

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