feat(net/goai): add configurable type mapping for oai type

[StrangeYear]

Summary

This PR adds a configurable type mapping mechanism in goai so specific Go types can be generated as custom OpenAPI schema types.
This solves cases like carbon.Carbon, which should be emitted as string instead of object.

Changes

• added Config.TypeMapping map[string]string in net/goai
• applied type mapping in golangTypeToOAIType before default type inference
• supported both mapping key formats:
  • short type name, e.g. carbon.Carbon
  • full type id, e.g. github.com/golang-module/carbon/v2.Carbon
• added unit test Test_TypeMapping to verify mapped struct type is emitted as string

Usage Example

oai := goai.New()
oai.Config.TypeMapping = map[string]string{
	"carbon.Carbon":                              goai.TypeString,
	"github.com/golang-module/carbon/v2.Carbon": goai.TypeString,
}