fix: mark optional Organization fields as nullable in OpenAPI spec (#1090)
Better Auth's OpenAPI generator does not emit nullable: true for fields
with required: false. This causes the Organization (Workspace) schema
to declare logo, metadata, and description as non-nullable strings,
while the API actually returns null for these fields.
Add a markOptionalSchemaFieldsNullable post-processor that walks
component schemas and adds nullable: true to properties not listed in
the required array.
Closes #1087
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