fix: add currentStepEstimate to required fields in JSON schema

OpenAI strict mode requires ALL properties to be in the required array.
The schema had currentStepEstimate property but wasn't marking it required.

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Co-Authored-By: Claude <noreply@anthropic.com>

apps/web/src/lib/ai/gradeWorksheet.ts

@@ -87,6 +87,7 @@ const gradingSchema = {
     'correctCount',
     'accuracy',
     'errorPatterns',
+    'currentStepEstimate',
     'suggestedStepId',
     'reasoning',
     'feedback',