…enapi-generator

following principles of 1. What went wrong and 2. how to solve it

…External-Codecs

refactor: improve the error message for missing external codecs in openapi-generator

Previously, only implicit-return arrow functions were supported:

```typescript
const factory = () => t.string;  // ✅ Worked

BlockStatement arrow functions with explicit returns would fail:
  const factory = () => {
  return t.string;  // ❌ Error: "BlockStatement arrow functions are not yet supported"
};
```

This fix:
- Searches BlockStatement.stmts for ReturnStatement nodes
- Extracts the return value expression (returnStmt.argument)
- Passes it to parseCodecInitializer() for recursive parsing
- Validates that a return statement exists with a non-undefined argument

Both styles now produce identical schemas since they converge on the
same parseCodecInitializer() call with the return value expression.

Test case added using BooleanFromNullableWithFallback() with explicit
block syntax to verify the parser handles both arrow function styles.

…-to-parse-utils

fix: support BlockStatement arrow functions in codec parser