Add ExecutionPolicy Bypass for powershell hooks

[pwang347]

Fixes #315830

[Copilot]

Pull request overview

This PR updates the Copilot extension’s hook execution on Windows to explicitly invoke PowerShell with -ExecutionPolicy Bypass when the system shell is cmd.exe, aligning hook behavior with environments where PowerShell execution policy would otherwise block hook scripts.

Changes:

• Refactors hook process creation to derive { command, args, shell, env } via a new getShellCommand(...) helper.
• On Windows + ComSpec=cmd.exe, runs powershell.exe directly with -ExecutionPolicy Bypass (and disables PowerShell update checks).
• Adds unit tests covering shell selection behavior across Windows/non-Windows scenarios.

Show a summary per file

File	Description
extensions/copilot/src/platform/chat/node/hookExecutor.ts	Refactors hook spawning to select a shell strategy and adds PowerShell -ExecutionPolicy Bypass for the Windows cmd.exe case.
extensions/copilot/src/platform/chat/test/node/hookExecutor.spec.ts	Adds tests asserting the selected shell/command/args for Windows vs non-Windows hook execution.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 3