[Studio] Validate consumer reset offset requests

[Aias00]

Summary

• replace ad hoc reset-offset request map parsing with a typed DTO
• validate required consumer group name and positive reset timestamp before invoking metadata admin operations
• return structured 400 responses for missing fields and invalid timestamp JSON types

Scope

• RocketMQ Studio contest scope: Track 1 / BASE-01 Consumer Group reset offset baseline hardening
• Does not introduce Namespace behavior

Tests

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -Dtest=ConsumerGroupControllerTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn test
• git diff --check

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[Aias00]

Superseded by #634. Consolidated with related instance resource action validation changes to reduce review overhead while preserving the same scoped fixes and tests.