Fix 1: Set Content-Type on all responses

In generateWriteProtoMessageResponseFunc (around line 860-893), the generated writeProtoMessageResponse function writes response bytes but never sets the Content-Type header. Fix this by adding w.Header().Set("Content-Type", ...) BEFORE w.WriteHeader(statusCode) (headers must be set before WriteHeader is called in Go's net/http).

The Content-Type should match the serialization format:

• JSON path: w.Header().Set("Content-Type", "application/json")
• Binary/Proto path: w.Header().Set("Content-Type", "application/x-protobuf")
• Default path (already defaults to JSON): w.Header().Set("Content-Type", "application/json")

Restructure the generated code so the Content-Type header is set inside the switch cases (or set a variable and set it after the switch but before WriteHeader). The cleanest approach:

var responseBytes []byte
var err error
var respContentType string
 
switch filterFlags(contentType) {
case JSONContentType:
    responseBytes, err = protojson.Marshal(msg)
    respContentType = "application/json"
case BinaryContentType, ProtoContentType:
    responseBytes, err = proto.Marshal(msg)
    respContentType = "application/x-protobuf"
default:
    responseBytes, err = protojson.Marshal(msg)
    respContentType = "application/json"
}
 
if err != nil {
    http.Error(w, fallbackMsg, statusCode)
    return
}
 
w.Header().Set("Content-Type", respContentType)
w.WriteHeader(statusCode)
_, _ = w.Write(responseBytes)

Fix 2: marshalResponse should default to JSON for unknown content types

In generateBindingHelpers (around line 629-653), the generated marshalResponse function has a default case that returns an error: return nil, fmt.Errorf("unsupported content type: %s", contentType). This is inconsistent with the writeProtoMessageResponse function which already defaults to JSON.

Change the default case in marshalResponse to default to JSON (matching protojson as source of truth):

default:
    // Default to JSON for unrecognized content types
    if marshaler, ok := response.(json.Marshaler); ok {
        return marshaler.MarshalJSON()
    }
    return protojson.Marshal(msg)

Also set the response Content-Type header in the handler function that calls marshalResponse. Look for where marshalResponse is called (in the generated handler functions) and add w.Header().Set("Content-Type", ...) there too. Find the generateMethodHandler or equivalent function that calls marshalResponse and writes the response. The Content-Type should be determined from the request Content-Type (matching the serialization format used) and set on the response.

Actually, check the full flow: marshalResponse returns bytes but doesn't have access to w. The caller writes the bytes. Find where the caller writes and add Content-Type there. The pattern is likely:

responseBytes, err := marshalResponse(r, response)
// ... error handling ...
w.WriteHeader(http.StatusOK)
w.Write(responseBytes)

Add Content-Type setting before WriteHeader. Determine the Content-Type by checking r.Header.Get("Content-Type") (same logic as marshalResponse) and set it on the response:

respContentType := "application/json"
if ct := filterFlags(r.Header.Get("Content-Type")); ct == BinaryContentType || ct == ProtoContentType {
    respContentType = "application/x-protobuf"
}
w.Header().Set("Content-Type", respContentType)

IMPORTANT: Do NOT change field names, function signatures, or break any public API. These are internal generated helper functions. The fix only adds Content-Type header setting and changes the default case from error to JSON. Run make build to verify compilation. Run UPDATE_GOLDEN=1 go test ./internal/httpgen/ -run TestExhaustiveGoldenFiles -count=1 to update golden files. Run go test ./internal/httpgen/ -count=1 to confirm all tests pass. Inspect the golden file diff to verify Content-Type headers are set and the default case is changed. Every generated HTTP server response includes a Content-Type header matching the serialization format (application/json or application/x-protobuf). Unknown request content types default to JSON serialization instead of returning an error. Golden files updated to reflect the changes.