fix(shell): isolate execution context for exec/cos commands

[AshokThangavel]

This PR introduces namespace isolation for the exec (and cos) command utility.

Currently, the exec command allows a user to execute arbitrary ObjectScript that can modify the process state. If a user executes a namespace switch (e.g., exec zn "%SYS"), the shell process remains in that new namespace context after the command completes. If that target namespace does not have the %IPM package mapped, the shell immediately crashes when it attempts to render the next prompt or error message because it can no longer find its own configuration classes.

Related Issues

• Fixes IPM exec Command Fails in Namespaces Without IPM Mappings After Execution #995

Testing Conducted

1. Namespace Drift Test: Ran exec zn "%SYS" from the USER namespace.

• Result: The command executed, and the shell remained in (or returned to) the USER namespace. The prompt rendered correctly.

2. Error Handling Test: Ran exec set x = 1/0 (divide by zero) to trigger an error.

• Result: The error was caught, and the namespace was still correctly restored.

3. Logic Verification: Confirmed that standard exec commands (like exec write 123) still function normally with no overhead.