[Studio] Validate instance delete requests

[Aias00]

Summary

• replace instance delete map parsing with a typed request DTO
• validate required instance ids before invoking delete operations
• add controller tests for blank and missing delete ids

Scope

• RocketMQ Studio contest scope: Track 1 / ARCH-01 instance access management hardening
• Does not introduce Namespace behavior

Tests

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

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[Aias00]

Superseded by #634. Folded into the consolidated instance resource action validation PR to keep related instance request validation fixes in one review unit.