fix(acl): align ACL 2.0 binding type validation with the policy service

[yyqdbngt]

Summary

• align Acl2PolicyContext.validate() with the operational AclService#validateAcl2Policy binding-type vocabulary
• require a non-blank boundType and accept TOPIC, GROUP, *, USER, SERVICE_ACCOUNT case-insensitively
• add regression coverage for the model validator's accepted and rejected binding types

Why

Two validators for the same ACL 2.0 policy concept disagreed: the model validator only accepted exactly-cased USER/GROUP/SERVICE_ACCOUNT and silently let a null boundType through, while the service validator (covered by AclServiceTest) requires a non-blank type and accepts the wider TOPIC/GROUP/*/USER/SERVICE_ACCOUNT set case-insensitively. A policy the service accepts ("topic", "user") failed the model's validate() with a misleading exception, and a null binding type passed the model but failed the service.

Testing

• cd server && mvn -q -Dtest=Acl2PolicyContextTest test — 11 tests pass (new class)
• cd server && mvn -q -Dtest=AclServiceTest test — all tests pass (regression for the aligned vocabulary)

[RockteMQ-AI]

LGTM. Clean fix that properly aligns the model validator with the service validator's binding-type vocabulary.

Observations

• Correctness ✓ — The null/blank guard (boundType == null || boundType.trim().isEmpty()) correctly prevents the previous silent pass-through of null values.
• Case-insensitive matching via toUpperCase(Locale.ROOT) is the right approach — locale-safe and consistent with the service layer.
• Tests ✓ — Good parameterized coverage for both accepted and rejected types, including edge cases like whitespace-only strings.

Minor Note

• java.util.Locale is fully qualified inline — consider adding an import for consistency with the rest of the file, but this is purely stylistic.

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