fix(webview): increase focus retry count for copyImage

[RITHIKKUMARAN]

Fixes #199981

Description

This PR addresses the long-standing issue where users are unable to copy images to their system clipboard via the right-click context menu inside webviews (such as Markdown Previews and Notebook outputs).

Root Cause:
When the VS Code custom context menu is invoked via a right-click, the webview iframe loses document focus. Because navigator.clipboard.write() strictly requires an active, focused document, it fails with a NotAllowedError if invoked while the context menu still holds focus. The existing code attempted to wait for focus to return, but the retry limit was set to only 5 retries at very short intervals (100-250ms total), which is frequently insufficient for the UI to return focus to the webview.

Proposed Changes:

• Increased the retries parameter in webviewPreloads.ts (Notebooks) from 5 to 50.
• Increased the retries parameter in preview-src/index.ts (Markdown) from 5 to 50.

This gives the webview up to 1 - 2.5 seconds to regain focus before executing the clipboard write operation, completely mitigating the timing race condition without introducing any blocking delays.

How to test

1. Open a Markdown file with an embedded image, or a Notebook with an image output.
2. Right-click the image to open the context menu.
3. Select "Copy Image".
4. Paste the image into an external program (like Paint or a browser).
5. The image should successfully paste instead of failing silently.

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[RITHIKKUMARAN]

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