fix: generate correct type for $ref text responses in strict server (oapi-codegen#2190)

When a response is defined as a $ref to a component response with
text/plain content type (and no headers), the strict-server template
generated a struct-embedding type whose Visit method tried to call
[]byte(response) on a struct, which failed to compile.

The root cause was the revert of PR oapi-codegen#1132 (commit 891a067), which had
originally fixed this by making all text responses string types. That
PR was reverted because it broke text responses with headers (oapi-codegen#1676),
which require the struct form with a Body field.

The fix extends the existing multipart special case in Branch 1A of the
strict-interface template to also cover text responses without headers.
This generates a named type alias (e.g. `type GetTest401TextResponse
UnauthorizedTextResponse`) instead of a struct embedding, so
[]byte(response) compiles and works correctly.

Text responses with headers continue to go through Branch 1C (struct
with Body + Headers fields), so oapi-codegen#1676 is unaffected — confirmed by
verifying no diff in internal/test/issues/issue-1676/ping.gen.go.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

internal/test/issues/issue-2190/issue2190.gen.go

@@ -14,7 +14,7 @@ import (
 // where the generated code tried to do []byte(response) on a struct type,
 // which does not compile.
 func TestGetTest401TextResponse(t *testing.T) {
-	resp := GetTest401TextResponse{UnauthorizedTextResponse("Unauthorized")}
+	resp := GetTest401TextResponse("Unauthorized")
 	w := httptest.NewRecorder()
 
 	err := resp.VisitGetTestResponse(w)

internal/test/issues/issue-2190/issue2190_test.go

@@ -40,7 +40,7 @@
         {{range .Contents}}
             {{$receiverTypeName := printf "%s%s%s%s" $opid $statusCode .NameTagOrContentType "Response"}}
             {{if and $fixedStatusCode $isRef -}}
-                {{ if and (not $hasHeaders) ($fixedStatusCode) (.IsSupported) (eq .NameTag "Multipart") -}}
+                {{ if and (not $hasHeaders) ($fixedStatusCode) (.IsSupported) (or (eq .NameTag "Multipart") (eq .NameTag "Text")) -}}
                 type {{$receiverTypeName}} {{$ref}}{{.NameTagOrContentType}}Response
                 {{else if $isExternalRef -}}
                 type {{$receiverTypeName}} struct { {{$ref}} }