[Studio] Validate Prometheus source queries

[Aias00]

Summary

• reject blank PromQL and step values in the Prometheus metrics source
• reject non-positive query start/end values before calling Prometheus
• add source-level regression coverage for invalid metric query input

Validation

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q -Dtest=PrometheusMetricsSourceTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q test (295 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Superseded by #637.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.