[BUG] Can't combine enum_value with empty_behavior #251

Describe the Bug

A message can use at most one JSON-customizing feature. Each feature's Go generator emits its own top-level MarshalJSON/MarshalJSONSebuf on the message, and two would be a duplicate method declaration, so the generators fail fast via pairwise conflict guards instead of composing. This makes a common, legitimate shape impossible: a message with string-mapped enums (enum_value) and nullable scalars (nullable or empty_behavior). The same exclusion applies between any two of: enum_value, int64_encoding=NUMBER, bytes_encoding, timestamp_format, nullable, empty_behavior, flatten, oneof_config.

To Reproduce

1. Create a proto with an enum that carries (sebuf.http.enum_value) mappings, and a message that uses that enum and a nullable (or empty_behavior) field (see Protobuf Definition below).
2. Run buf generate.
3. See the error under Actual Behavior.

Note: this is not fixable by changing the field's enum_encoding. The enum marshaler is triggered by the enum type carrying enum_value mappings, not by the field annotation:

• Dropping ENUM_ENCODING_STRING leaves the default (still not NUMBER) → enum marshaler still active → same conflict.
• Setting ENUM_ENCODING_NUMBER → rejected by a separate validation: "field ... has both enum_encoding=NUMBER and enum_value annotations - this is not allowed" (internal/httpgen/enum_encoding.go:19).

Expected Behavior

Both features coexist on one message — type serializes as "call" and delta serializes as null when unset:

{ "type": "call", "delta": null }

Actual Behavior

buf generate aborts:

Failure: plugin protoc-gen-go-http: message OptionsDetails: enum_value requires MarshalJSON
but conflicts with nullable (also requires MarshalJSON) -- only one MarshalJSON-generating
feature is supported per message

(Using empty_behavior instead of nullable yields the identical error with empty_behavior named. protoc-gen-go-client has a mirror guard, internal/clientgen/enum_validation.go:15.)

Environment (please complete the following information):

• OS: macOS 26.6.1 (Apple Silicon / arm64)
• Go version: go1.26.5
• sebuf version: generator built from main @ d3bcc2a; annotations module buf.build/sebmelki/sebuf commit b5f679ca6c5f4f148c3414adfea268d3
• protoc version: n/a (generating via buf)
• buf version: 1.72.0

Protobuf Definition

syntax = "proto3";

import "sebuf/http/annotations.proto";

enum OptionType {
  OPTION_TYPE_UNSPECIFIED = 0;
  OPTION_TYPE_CALL = 1 [(sebuf.http.enum_value) = "call"];
  OPTION_TYPE_PUT  = 2 [(sebuf.http.enum_value) = "put"];
}

message OptionsDetails {
  OptionType type = 1;                                       // string-mapped enum
  optional double delta = 2 [(sebuf.http.nullable) = true];  // nullable scalar
}

Generated Code (if applicable)

None — generation aborts before emitting. The conflict is that both features would emit the same method on OptionsDetails:

// from the enum_value marshaler (enum_field_encoding.go)
func (x *OptionsDetails) MarshalJSONSebuf(opts protojson.MarshalOptions) ([]byte, error) { /* ... */ }
func (x *OptionsDetails) MarshalJSON() ([]byte, error) { /* ... */ }

// AND from the nullable marshaler (nullable.go) — duplicate declaration on the same type
func (x *OptionsDetails) MarshalJSONSebuf(opts protojson.MarshalOptions) ([]byte, error) { /* ... */ }
func (x *OptionsDetails) MarshalJSON() ([]byte, error) { /* ... */ }

Command Used

buf generate

Additional Context

Root cause (paths at main @ d3bcc2a):

• Each feature emits its own MarshalJSON/MarshalJSONSebuf: internal/httpgen/empty_behavior.go:131, plus nullable.go, enum_field_encoding.go, encoding.go (int64), bytes_encoding.go, timestamp_format.go, flatten.go, oneof_discriminator.go.
• Pairwise fail-fast guards: enum_field_encoding.go:261 checkEnumMarshalJSONConflict, mirrored in encoding.go:211, flatten.go, oneof_discriminator.go.
• Enum trigger is the mapping on the enum type: enum_field_encoding.go:49 customEnumForField.

Workaround: for a message that needs string-mapped enums, keep nullable scalars as google.protobuf.DoubleValue/Int32Value wrappers — protojson renders unset wrappers as null natively without a message-level MarshalJSON, so they don't hit the guard.