[cherry-pick] ChatSessionCustomizationProvider testing fixes

[vs-code-engineering]

Cherry-pick of #308071 from main.

Fixes several issues found during testing of the ChatSessionCustomizationProvider API (testing issue microsoft/vscode-internalbacklog#7330).

Commits

• Delete individual hooks instead of entire hooks file — Uses JSONC removeProperty/applyEdits to surgically remove a single hook entry while preserving comments. Passes itemId in both inline and right-click context menus. Wraps in try/catch with error dialog for stale file contents. Fixes microsoft/vscode-internalbacklog#7381
• Throw CancellationError when plugin trust dialog is declined — Properly throws so the catch handler resets the button to "Install" instead of showing "Installed". Updates 2 existing tests. Fixes microsoft/vscode-internalbacklog#7378
• Check installed state when rendering marketplace plugin items — Uses isEqual URI comparison against IAgentPluginService.plugins. Also fixes the PluginBrowseMode fixture (was broken due to missing ICustomizationHarnessService mock). Fixes microsoft/vscode-internalbacklog#7379
• Replace floppy disk icon with newFile icon for create plugin button — Fixes microsoft/vscode-internalbacklog#7373
• Fix plugin page CSS: padding, back button, and focus outlines — microsoft/vscode-internalbacklog#7377, microsoft/vscode-internalbacklog#7384, microsoft/vscode-internalbacklog#7385
• Show workspace folder name in provider path for multi-root workspaces — Uses labelService.getUriLabel() for file: URIs instead of basename(). Fixes microsoft/vscode-internalbacklog#7330

Companion PR: microsoft/vscode-copilot-chat#5005