Fix duplicate content-type header bug, add test

[wfhartford-wordly]

Micronaut adds a second Content-Type header to an HttpResponse when the result of the controller function is a HttpResponse<Flux<String>> (or other nested data type), and the controller explicitly sets the content type on the HttpResponse. This can be demonstrated with the following simple controller:

@Controller
class VariableContentTypeController {
	@Get("/")
	fun index(): HttpResponse<Flux<String>> {
		return HttpResponse.ok(Flux.just("Hello World!")).contentType("text/plain")
	}
}

See https://github.com/wfhartford-wordly/micronaut-duplicate-content-type-headers for a full project demonstrating the bug.

Calling this function from curl shows that two content type headers are present in the response.

$ curl localhost:8080 -v
* Host localhost:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:8080...
* Connected to localhost (::1) port 8080
> GET / HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/1.1 200 OK
< Content-Type: text/plain
< Content-Type: text/plain
< date: Thu, 15 Jan 2026 23:45:04 GMT
< transfer-encoding: chunked
< 
* Connection #0 to host localhost left intact
Hello World!

This PR fixes the bug by changing the RouteExecutor to call the contentType function which sets the header rather than the header function which adds it. I've also included a unit test which verifies the fix.

I first encountered this bug in Micronaut version 4.3.8 and have confirmed that it is present in version 4.10.7 and on the 5.0.x branch.

[cla-assistant]

All committers have signed the CLA.

[graemerocher]

do you wish to target the 4.10.x branch or keep this in 5.0.x?

[wfhartford-wordly]

do you wish to target the 4.10.x branch or keep this in 5.0.x?

I'd love to see this fixed in 4.10.x. Would you like me to create another PR targeting that branch?

[graemerocher]

@wfhartford-wordly a PR targeting 4.10.x would make sense. Thanks

[wfhartford-wordly]

#12376 Is the same change targeting the 4.10.x branch.