Add test for BOM character preservation in VSBuffer (issue #251527)

[Copilot]

Adds regression test for PR #272389 which fixed files with BOM (\uFEFF) characters in filenames failing to open. The bug occurred because TextDecoder was stripping BOM during IPC message deserialization.

Test Coverage

The test verifies VSBuffer.toString() preserves BOM characters:

test('issue #251527 - VSBuffer#toString preserves BOM character in filenames', () => {
    const bomChar = '\uFEFF';
    const filename = `${bomChar}c.txt`;
    const buffer = VSBuffer.fromString(filename);
    const result = buffer.toString();

    assert.strictEqual(result, filename);
    assert.strictEqual(result.charCodeAt(0), 0xFEFF);
});

Without the fix (ignoreBOM: true in TextDecoder constructor), this test fails as the BOM is stripped from the filename.

Fixes #251527

Original prompt

Add a test for the fix in #251527

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[Copilot]

Pull Request Overview

This PR adds a regression test for issue #251527, which involved files with BOM (\uFEFF) characters in filenames failing to open due to TextDecoder stripping the BOM during IPC message deserialization. The test validates that VSBuffer.toString() now preserves BOM characters after the fix (adding ignoreBOM: true to the TextDecoder constructor).

Key Changes

• Added test case issue #251527 - VSBuffer#toString preserves BOM character in filenames to verify BOM preservation
• Test creates a buffer from a string containing BOM character and validates it's preserved through serialization/deserialization

[bpasero]

@microsoft-github-policy-service rerun