fix: prevent terminal hangs when PowerShell prompt fails

[xinyi-gong]

Summary

Fixes Agent mode hanging indefinitely after a terminal command completes on Windows.

Root cause

The PowerShell integration invoked the original prompt using ScriptBlock.Invoke(). This method call is blocked in ConstrainedLanguage mode. A user-defined prompt could also throw an exception or return no output.

When the prompt failed, the shell integration did not emit its prompt-end marker. Consequently, the terminal command future never completed and Agent mode remained stuck on Running run_in_terminal tool.

Changes

• Execute the original prompt using PowerShell's call operator instead of .Invoke().
• Catch failures from user-defined prompt functions.
• Fall back to a standard prompt containing the current directory when the original prompt fails or returns no output.
• Ensure shell completion markers continue to be emitted.

Validation

• Reproduced the hang in ConstrainedLanguage mode.
• Verified terminal commands complete after the fix in ConstrainedLanguage mode.
• Verified fallback behavior when a custom prompt:
  • throws an exception;
  • returns $null.
• Verified normal PowerShell prompt behavior remains unchanged.
• .\mvnw checkstyle:check passed.
• .\mvnw test passed.

Fixes #375

[Copilot]

Pull request overview

This pull request fixes a Windows-specific Agent mode hang by hardening the PowerShell shell-integration prompt handling so the tool completion markers are still emitted even when the user’s prompt fails (notably in ConstrainedLanguage mode).

Changes:

• Invoke the original PowerShell prompt via the call operator (&) instead of ScriptBlock.Invoke() to avoid ConstrainedLanguage blocking.
• Guard prompt execution with try/catch and suppress prompt errors to prevent breaking the integration flow.
• Provide a fallback prompt (current directory) when the original prompt throws, returns $null, or returns an empty string.