Add unit tests for core extension functionality

[Copilot]

The extension had no proper test coverage. This PR adds 20 unit tests covering the testable pure functions and data structures.

Tests Added

Utils (8 tests)

• stripAnsi() - ANSI escape code removal
• isLibPathWriteable() - directory permission checking
• waitForFile() - async file existence with timeout

Sidebar (7 tests)

• SidebarProvider - package storage, filtering, max length calculations
• RPackageItem - tree item creation for System/User/renv package types
• Fuzzy search and placeholder display logic

Events (1 test)

• Code pattern detection for library(), require(), p_load(), detach()

Example

test('stripAnsi removes ANSI escape codes', () => {
  const textWithAnsi = '\x1b[31mError:\x1b[0m Something went wrong';
  const cleanText = stripAnsi(textWithAnsi);
  assert.strictEqual(cleanText, 'Error: Something went wrong');
});

test('RPackageItem creates proper tree item for renv package', () => {
  const pkg: RPackageInfo = {
    name: 'ggplot2',
    version: '3.4.0',
    libpath: '/project/renv/library',
    locationtype: 'renv',
    title: 'Grammar of Graphics',
    loaded: false
  };

  const item = new RPackageItem(pkg, 0, 0);
  assert.strictEqual(item.label, 'ggplot2');
  assert.ok(typeof item.description === 'string' && item.description.includes('renv'));
});

Tests focus on pure functions without R runtime or VS Code extension host dependencies for fast, reliable execution.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add tests</issue_title>
<issue_description>At the moment, we don't have any proper tests. Let's ask copilot to see if we can come up with any useful tests.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add tests #50

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