fix(vscode): support executable paths containing spaces

[Vamsi-klu]

Description

Fixes #6093.

• Replace shell-based command construction with execFile(command, args) so executable paths and arguments containing spaces are passed intact.
• Preserve ExecResult behavior for nonzero exits and spawn failures while continuing to reject cancellation with AbortError.
• Pass uv test-helper arguments separately and add regression coverage for spaced paths, literal shell metacharacters, trace output, failures, and cancellation.

Test Plan

• pnpm run test:unit — 26 passed, including direct executable and argument coverage under paths containing spaces
• pnpm exec eslint src/utilities/exec.ts src/utilities/exec.test.ts tests/utils.ts — passed
• pnpm run check-types — passed
• node esbuild.js — extension bundle passed
• pnpm run vscode:package — VSIX packaging passed
• pnpm exec playwright test tests/diagnostics.spec.ts — 10 passed in 1.2m

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO

[Vamsi-klu]

@benfdking @georgesittas — could you review this? It touches the VS Code subprocess helper and E2E utilities you have previously worked on and reviewed.