Fix OpenAPI 3.1 schema errors in API spec

[AlexCatarino]

Summary

• Convert 46 nullable: true usages to OpenAPI 3.1 syntax (type: [T, "null"] for typed properties, oneOf with type: "null" for allOf/$ref properties)
• Fix type: int to type: integer in Project.maxFileSize
• Add 5 missing properties (created, psr, sharpeRatio, trades, cloneId) to Optimization schema and remove 2 non-existent fields (apiProviders, source) from its required array
• Regenerate API reference pages

Resolves #2297

Test plan

• Run vacuum lint QuantConnect-Platform-2.0.0.yaml --fail-severity error and confirm zero schema errors
• Verify API reference pages render correctly on quantconnect.com/docs
• Confirm generated pages match the updated YAML spec