Fix: Serialize request body before validation to support custom types

Problem

When using custom transformers (introduced in #2281 and #2383) with types like Decimal, there was a type mismatch between the TypeScript interface and the OpenAPI schema validation:

• OpenAPI Schema: Defines the field as string with format decimal
• TypeScript Type: Uses custom type Decimal (generated via typeTransformers)
• Runtime Value: Contains a Decimal instance before serialization

This caused validation failures because the validator expected a string but received a Decimal object.

Solution

Reordered the request processing pipeline to serialize before validate:

Before:

if (opts.requestValidator) {
  await opts.requestValidator(opts); // ❌ Validates Decimal object against string schema
}

if (opts.body && opts.bodySerializer) {
  opts.body = opts.bodySerializer(opts.body); // Converts Decimal to string
}

After:

if (opts.body && opts.bodySerializer) {
  opts.body = opts.bodySerializer(opts.body); // ✅ Converts Decimal to string first
}

if (opts.requestValidator) {
  await opts.requestValidator(opts); // ✅ Validates serialized string
}

Usage

With this fix, users can now use custom types with proper serialization:

// Client configuration
const client = createClient({
  bodySerializer: (body: any) => JSON.parse(JSON.stringify(body))
});

// Works correctly with custom types
const response = await client.POST('/api/endpoint', {
  body: {
    amount: new Decimal('123.45') // Serialized to "123.45" before validation
  }
});

This ensures compatibility between custom TypeScript types and OpenAPI schema validation while maintaining type safety throughout the request lifecycle.