fix: Fix command run failure in the integrated terminal

Description

Fixes terminal manager hanging on subsequent command executions after initial shell integration failures. The root cause was terminals being cached before shell integration was ready combined with missing error handling in the RPC handler that prevented the Language Server from receiving error responses.

• This fix ensures terminals are only cached after shell integration is confirmed
• adds error handling to always return responses
• properly disposes event listeners to prevent leaks
• re-validates cached terminals before execution

Related Issues

References #2139

How has this been tested?

1. Run extension from this MR in the extension dev host
2. Ask chat to run commands
3. Approve
4. Command should execute successfully in the integrated terminal (if available).

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes