Fix to handle keyword properly

[h-rajagopal]

This pull request introduces support for the standard JPA CAST function syntax with the AS keyword, alongside tests to validate the new functionality. It also removes the unused UNION keyword from the grammar. Below is a summary of the most important changes:

Enhancements to CAST Functionality

• Updated the CAST function grammar in Eql.g4 to support the standard JPA syntax: CAST(expression AS type) with optional numeric literals for precision.
• Modified EqlQueryRenderer.java to handle the new CAST syntax while maintaining backward compatibility with the original EclipseLink syntax.

Grammar Cleanup

• Removed the unused UNION keyword from the Eql.g4 grammar.

Testing

• Added a new integration test class, CastFunctionIntegrationTests, to verify the standard JPA CAST syntax in real-world scenarios.
• Added test cases in EqlComplianceTests to validate the CAST function with various data types and precision.

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