…api-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>