Fix output policy validation in UserCodeService.is_execution_allowed

[Nitya-Choudhary]

Fixes #9391

What changed

• Updated UserCodeService.is_execution_allowed() to respect the boolean return
  value of output_policy.is_valid().
• Execution is now allowed only when is_valid() returns True.
• Execution is correctly rejected when is_valid() returns False or raises
  an exception.

Why this change is needed

Previously, output_policy.is_valid() was called only for its side effects.
If it returned False, execution was still allowed, which bypassed output
policy enforcement. This fix ensures output policies are properly enforced
as intended.

Tests added

• Added unit tests to verify execution is allowed for valid output policies.
• Added unit tests to verify execution is rejected for invalid output policies.

Impact

• Improves correctness and security of output policy validation.
• No breaking changes to public APIs.