Properly handle the typeName of unions in Mappings API

[pasindufernando1]

Purpose

Addresses : wso2/product-ballerina-integrator#1143

[Copilot]

Pull Request Overview

This PR addresses proper handling of union type names in the Mappings API by ensuring they display the constituent member types rather than generic or empty type names.

• Updates union type name generation to concatenate member type names with pipe separators
• Changes nil type representation from "nil" to "()" for consistency
• Updates test data files to reflect the corrected union type name formatting

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

File	Description
ReferenceType.java	Changes nil type name representation from "nil" to "()"
DataMapManager.java	Implements logic to build union type names from member type names
union*.json, enum*.json, variable*.json	Updates test data to reflect proper union type name formatting

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