[Studio] Cover metrics query request validation

[Aias00]

Summary

• add MetricsController coverage for blank PromQL expressions, non-positive start timestamps, and blank query steps
• assert invalid requests return 400 before reaching the metrics service

Validation

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q -Dtest=MetricsControllerTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q test (50 surefire files, 296 tests, 0 failures, 0 errors)
• git diff --check

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[Aias00]

Superseded by #637. Consolidated with related Metrics/Prometheus validation changes to reduce review overhead while preserving the same scoped fixes and tests.