fix: highlight active match when opening find/replace widget

[kiofaw]

fix: #15067
Fixes the issue where the current match is not visually highlighted when opening the Find (Ctrl+F) or Find and Replace (Ctrl+H) widget.

• When find matches are computed, immediately highlight the current match (the one at or after the cursor position)
• Pass the actual currentMatch range to changeMatchInfo() instead of undefined
• This ensures the active match is always visually indicated when the find widget is opened

Unit tests updated in findModel.test.ts

Before

actual.mp4

After

expected.mp4

[kiofaw]

@microsoft-github-policy-service agree