markdown: Reset preview scroll for new files

[Dmitriy Vasyura (dmitrivMS)]

• reset persisted Markdown preview scroll progress when opening a different file without an explicit source line
• preserve source-line and fragment-based preview navigation

Fixes #164071

Validation

• npm run build-webview --prefix .\extensions\markdown-language-features
• Code OSS: Explorer Open Preview resets a different file to the top
• Code OSS: plain relative links reset the linked file to the top
• Code OSS: relative fragment links reveal the requested heading
• Code OSS: reopening or hiding/revealing the same preview preserves its scroll position
• Code OSS: opening a long file from source line 0 starts at the top (regression from Markdown scrolls to bottom of doc #168223)
• Code OSS: explicit source-line navigation wins over stale scroll progress, both when opening to the side and when an unlocked preview switches resources

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: None

What changed in this PR

Resets Markdown preview scrolling when navigating to a different file while preserving explicit line and fragment navigation.

Changes:

• Resets persisted scroll progress to the top for newly opened files.
• Retains existing source-line and fragment behavior.

File	Description
extensions/​markdown-language-features/​preview-src/​index.ts	Resets stale preview scroll progress when switching resources.

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