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 and shared-environment warm-up arguments separately and add regression coverage for spaced paths, literal shell metacharacters, trace output, failures, and cancellation.

Test Plan

• make style: all four checks passed.
• make fast-test: all six phases passed; the main phase reported 2,699 passed and 6 skipped.
• Root pnpm run lint: passed across the workspace.
• Extension unit tests: 28 passed, including the shared-environment warm-up regression.
• TypeScript checks and extension bundle: passed.
• VSIX packaging: passed.
• Packaged diagnostics browser tests: 10 passed with one worker.
• Real interpreter warm-up from a path containing spaces: passed.

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.