[Studio] Handle missing request parameters

[Aias00]

Summary

• return structured 400 responses when required request parameters are missing
• cover the missing @RequestParam path in GlobalExceptionHandlerTest

Validation

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q -Dtest=GlobalExceptionHandlerTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -q test (292 tests, 0 failures, 0 errors)
• git diff --check

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[Aias00]

Superseded by #632. Folded into the consolidated request validation handling PR because missing request parameter handling is part of the same validation/error-response surface.