feat(go,typescript,csharp,python): add nested discriminated unions support (#1647)

## Release Summary

### Go
- Added support for nested discriminated unions with improved
discriminator validation that traverses associated union members to
lookup discriminator property fields
- Fixed unused imports from the `types` package by scoping imports only
to pointer types where needed

### TypeScript
- Enhanced discriminator resolution to handle union types by inspecting
union members to resolve discriminator fields and types
- Improved error messages for discriminator validation
- Applies to both TypeScript v2 and MCP TypeScript targets

### Python, Csharp
- Added support for nested discriminated unions

--- 

## Related
- feat: support nested discriminated unions
  > speakeasy-api/openapi-generation#3217

Author: vishalg0wda

go.mod

@@ -38,7 +38,7 @@ require (
 	github.com/sethvargo/go-githubactions v1.1.0
 	github.com/speakeasy-api/huh v1.1.2
 	github.com/speakeasy-api/openapi v1.7.3
-	github.com/speakeasy-api/openapi-generation/v2 v2.720.5
+	github.com/speakeasy-api/openapi-generation/v2 v2.721.0
 	github.com/speakeasy-api/openapi-overlay v0.10.3
 	github.com/speakeasy-api/sdk-gen-config v1.35.0
 	github.com/speakeasy-api/speakeasy-client-sdk-go/v3 v3.26.7

go.sum

@@ -541,8 +541,8 @@ github.com/speakeasy-api/libopenapi v0.21.9-fixhiddencomps-fixed h1:PL/kpBY5vkBm
 github.com/speakeasy-api/libopenapi v0.21.9-fixhiddencomps-fixed/go.mod h1:Gc8oQkjr2InxwumK0zOBtKN9gIlv9L2VmSVIUk2YxcU=
 github.com/speakeasy-api/openapi v1.7.3 h1:QM9VglcsxRPH19Xr47nYRtDsKOlczCdRRxPYa0pAGz0=
 github.com/speakeasy-api/openapi v1.7.3/go.mod h1:fy+CvRcKj+HDU0QNdnyG6UkfJOEjhqCuNC7o4AtLPAk=
-github.com/speakeasy-api/openapi-generation/v2 v2.720.5 h1:ndAXRi5Qb0RO4k5CYUlkQbuG8AQdOtVbLO6KmXoZNqI=
-github.com/speakeasy-api/openapi-generation/v2 v2.720.5/go.mod h1:fy5/XuA7hfY9pJkesh8av1l6PoQT1KFjzLZe38OAYFI=
+github.com/speakeasy-api/openapi-generation/v2 v2.721.0 h1:dFfY9rMo7uWHr8SwXkEm/nUa1UWXsKnrE1TpW7PVaMo=
+github.com/speakeasy-api/openapi-generation/v2 v2.721.0/go.mod h1:fy5/XuA7hfY9pJkesh8av1l6PoQT1KFjzLZe38OAYFI=
 github.com/speakeasy-api/openapi-overlay v0.10.3 h1:70een4vwHyslIp796vM+ox6VISClhtXsCjrQNhxwvWs=
 github.com/speakeasy-api/openapi-overlay v0.10.3/go.mod h1:RJjV0jbUHqXLS0/Mxv5XE7LAnJHqHw+01RDdpoGqiyY=
 github.com/speakeasy-api/sdk-gen-config v1.35.0 h1:9yM5BwCqs+HmbyjIXwy+bciIdZROSJJWp98prFwe0OE=