Prevent malformed Markdown fences from freezing the Webview

[SivanCola]

Summary

• make Markdown block parsing guarantee forward progress for extended, unusual, and unclosed code fences
• preserve the long-chat synchronization, visibility, lightweight state, and bounded transcript implementation already merged in PR Fix VS Code freezes on long Reasonix chats #13
• add focused parser coverage plus an Extension Host regression with approximately 330 KiB / 206 chat items and a formerly poisonous fence
• handle the macOS VS Code 1.131 test executable rename in the Extension Host launcher

Root cause

The Webview Markdown loop did not advance when a line began with a fence marker but failed the strict opening-fence regular expression, such as a C# fence with attributes or a four-backtick fence. The paragraph branch rejected the same line, so the loop repeatedly appended empty paragraphs without advancing and could freeze the Webview.

Conflict resolution

This branch is rebased onto the overlapping long-chat fix in PR #13, the Windows launcher changes in PR #14, and the Webview theme adaptation in PR #11. Duplicate long-chat synchronization changes were removed. The combined test entry list retains the launcher, theme, and Markdown suites, while both Windows configured/PATH modes and the macOS VS Code executable fallback remain intact.

Validation

• npm ci: 0 vulnerabilities
• npm run lint
• npm test: 73/73 passed
• npm run test:vscode: VS Code 1.131.0 Extension Host passed in configured-binary mode
• npm run smoke:acp
• npm run package:vsix
• npm run verify:vsix-contents
• git diff --check

Closes #8