fix(worksheet-parsing): remove .optional() for OpenAI strict mode compatibility

OpenAI's strict JSON schema mode requires ALL properties to be in the
`required` array. The `notes` and `excluded` fields used `.optional()`
which caused them to be excluded from `required`, resulting in API error:
"Missing 'notes'" in required array.

Fix:
- `notes`: Changed from `.nullable().optional()` to just `.nullable()`
- `excluded`: Changed from `.optional()` to `.default(false)`

Both fields are now properly included in the required array while still
allowing null/false values as defaults.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/src/lib/worksheet-parsing/schemas.ts

@@ -144,10 +144,9 @@ export const ParsedProblemSchema = z
     notes: z
       .string()
       .nullable()
-      .optional()
       .describe(
         'Brief explanation if confidence is low. Examples: "Answer partially obscured by smudge", ' +
-          '"Digit could be 7 or 1", "Answer box appears empty". Null/omit if confidence is high.',
+          '"Digit could be 7 or 1", "Answer box appears empty". Null if confidence is high.',
       ),
 
     // Bounding boxes for UI highlighting
@@ -161,7 +160,7 @@ export const ParsedProblemSchema = z
     // User exclusion (set by corrections, not LLM)
     excluded: z
       .boolean()
-      .optional()
+      .default(false)
       .describe(
         "True if user marked this problem to be excluded from the session",
       ),