fix: Debounce metadata search filter to prevent UI freezing

[Exotic209093]

Summary

• Adds a 150ms debounce to the metadata filter input in the Download Metadata page
• The filter value updates immediately (so the input stays responsive), but the expensive DOM filtering is deferred until the user pauses typing

Root Cause

When many metadata sections are expanded (e.g., CustomMetadata + ApexClass), filtering runs on every keystroke and blocks the main thread, causing the input to freeze and typed characters to appear in bursts.

Test Plan

• Open Download Metadata, expand several large sections (e.g., CustomMetadata, ApexClass)
• Type quickly in the search filter — input should remain responsive
• Verify filtering still works correctly after the debounce delay
• Clear the filter and verify all items reappear

Closes #959