test(admin): add unit tests for VariableUtils

[gaoxiaolei-s59]

Why

spring-ai-alibaba-admin-server-core's VariableUtils implements the variable-resolution core used by workflow nodes — ${...} expression parsing, nested payload path get/set, workflow-context resolution (valueFrom input/refer), type coercion (convertValueByType), and variable identification in prompt text — but currently has no test coverage.

What

Adds VariableUtilsTests (32 tests, pure JUnit 5, no mocks) covering every public method:

• getExpressionFromBracket: wrapper stripping, plain expressions, blank input
• getValueFromPayload / getValueStringFromPayload: top-level and nested paths, missing paths, illegal-character expressions, scalar vs JSON stringification
• setValueForPayload: top-level/nested writes, invalid input
• getValueFromContext / getValueStringFromContext: input vs refer sources, blank valueFrom default, payload-expression fallback, null handling
• convertValueByType: String/Number/Boolean/Object/typed arrays, null passthrough, BizException on unconvertible values
• identifyVariableListFromText / identifyVariableSetFromText: ordering, duplicates, deduplication, no-variable text

Verification

mvn -pl spring-ai-alibaba-admin-server-core -am test -Dtest=VariableUtilsTests
Tests run: 32, Failures: 0, Errors: 0, Skipped: 0
BUILD SUCCESS