The generated JsonRpcResponseSchema incorrectly makes both result and error optional, allowing invalid responses where both fields are present or both are absent.

Root Cause:
The OpenAPI spec correctly uses oneOf to enforce that exactly one of result or error must be present. However, the code generator in tools/codegen/generate.ts lines 590-594 hardcodes a simplified schema that ignores this constraint.

Expected:
Per JSON-RPC 2.0 spec, a response MUST contain either result OR error, never both, never neither.

Actual:
Current schema allows:

• Both result and error present
• Neither result nor error present

Fix:
Use Zod's union types to properly implement the oneOf constraint from the OpenAPI spec.

Location:

• tools/codegen/generate.ts lines 590-594
• packages/jsonrpc-types/src/schemas.ts lines 5450-5456
• Test acknowledging this issue: packages/jsonrpc-types/src/__tests__/schemas.test.ts lines 145-165