[CDAP-21270] fix: restore JEXL allowlist explanation in error message

[sahusanket]

Restores the explanatory sentence Make sure the JEXL transformation is valid and uses only allowlisted classes, methods, and properties. to the JEXL evaluation error message in EL.handleJexlException (follow-up to #1055).

[gemini-code-assist]

Code Review

This pull request updates the JEXL evaluation error message in EL.java to suggest checking if the transformation is valid and uses only allowlisted classes, methods, and properties, and adds a corresponding assertion in RecipePipelineExecutorTest.java. Feedback suggests conditionally appending the allowlist mention only when allowlistEnabled is true to prevent misleading users when the allowlist is disabled.

[riyaa14]

Can we split the base message like I was doing before?

Here we can add "Make sure JEXL transformation is valid."

and above, we can add: "Make sure JEXL transformation uses only allowlisted classes, methods and properties."

[riyaa14]

Here, we can throw this message:

baseErrorMessage + "Make sure JEXL transformation uses only allowlisted classes, methods and properties."