fix: normalize Windows drive letter when matching test failure stacks

[afrasso]

Summary

• Fix Windows test failure squiggles not appearing when vitest.applyDiagnostic is enabled
• Normalize the test item file path with normalizeDriveLetter() in parseLocationFromStacks, matching how stack paths are already normalized

Fixes #795

Problem

On Windows, Vitest stack frames use an uppercase drive letter (C:\...) while VS Code's testItem.uri.fsPath uses lowercase (c:\...). parseLocationFromStacks only normalized the stack side, so the path comparison failed and the extension logged "Could not find a valid stack" instead of drawing a diagnostic squiggle.

Test plan

• On Windows, open a project with Vitest tests
• Enable vitest.applyDiagnostic (default)
• Run a failing test from the Testing panel
• Confirm a squiggle appears on the failing expect(...) line
• Confirm CI passes (pnpm typecheck, pnpm test, pnpm test-e2e)