Fix enum type misinterpretation as union in record fields

[pasindufernando1]

Purpose

This PR fixes the issue of the inability of the enum members of a record getting treated as unions in the data-mapper backend model. Here when a union type is encountered, all its members are checked if they are singleton (which is the case of enum) and if so it is treated as a enum.
Fixes : wso2/product-ballerina-integrator#1381

[Copilot]

Pull request overview

This pull request fixes an issue where enum types were being misinterpreted as union types when used in record fields. The fix introduces logic to detect when a union type consists entirely of singleton members, which indicates it should be treated as an enum type instead.

Changes:

• Added enum detection logic in the union type processing to identify when all members are singletons
• Refactored enum type creation into a reusable createEnumType helper method
• Updated test data files to reflect the correct enum representation (using "members" instead of "memberTypes" and "enum" instead of "union")

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
misc/diagram-util/src/main/java/org/ballerinalang/diagramutil/connector/models/connector/ReferenceType.java	Added singleton detection logic to differentiate enums from unions, and refactored enum creation into a helper method
misc/diagram-util/src/test/resources/RefTypeTest/TestData/enumSample1.json	Updated test data to use correct enum structure (members/enum) instead of union structure (memberTypes/union)
misc/diagram-util/src/test/resources/RefTypeTest/TestData/enumSample2.json	New test data file for validating enum type handling in records
misc/diagram-util/src/test/resources/RefTypeTest/BalProject7/main.bal	Added Color enum and Person2 record for testing enum field handling
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_model/config/enum.json	Updated member names to use qualified enum notation and changed type representation from union to enum
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_model/config/enum2.json	Updated member names to use qualified enum notation and changed type representation from union to enum
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_custom_function/config/enum_member.json	New test configuration file for enum member handling in custom functions
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_custom_function/source/proj4/main.bal	New test source file with function transforming optional patient data
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_custom_function/source/proj4/Ballerina.toml	New Ballerina project configuration for test project
flow-model-generator/modules/flow-model-generator-ls-extension/src/test/resources/data_mapper_custom_function/source/proj4/functions.bal	Empty functions file for test project

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[NipunaRanasinghe]

@pasindufernando1 could you please attach the valid issue link in the PR description? current link seems to be pointing to the issues tab