fix(api): emit bare interface for all sealed-interface unions

Optional fields whose type was a sealed-interface union without an
auto-decode discriminator (BotCommandScope, InputMedia, InputPaidMedia,
InputProfilePhoto, InputStoryContent, InputMessageContent,
InputPollMedia, InputPollOptionMedia, InlineQueryResult,
PassportElementError) were emitted as *<Union> — pointer to interface,
which is a Go anti-pattern: interfaces are already nil-able, and
callers were forced to take addresses of concrete variants.

The codegen path goType() guarded against pointer-wrapping using
knownDiscriminators (only 13 unions with auto-decode dispatch), missing
the 10 marker-only sealed interfaces. New package var
knownInterfaceTypes is built from buildUnionTypeSet at emitter
construction and covers both kinds. goType() now consults that.

Net effect:
- api/methods.gen.go: 3 *BotCommandScope and 2 *InputPollMedia fields
  become bare interface
- api/types.gen.go: 14 *InputMessageContent and 1 *InputPollOptionMedia
  fields become bare interface

Regression tests in api/unionparam_test.go cover both shapes:
direct concrete-variant assignment, and nil omitempty on the bare
interface field.

cmd/genapi/emitter.go

@@ -168,9 +168,18 @@ type emitter struct {
 }
 
 func newEmitter(api *spec.API, outDir string) *emitter {
+	knownInterfaceTypes = buildUnionTypeSet(api)
 	return &emitter{api: api, outDir: outDir, enums: planEnums(api)}
 }
 
+// knownInterfaceTypes is the full set of sealed-interface union type names
+// (both auto-decoded ones in knownDiscriminators and marker-only ones from
+// types with OneOf). Populated at emitter construction. goType and
+// unionTypeFor consult this so optional fields of any union type stay
+// bare interface, never *Interface (which is meaningless in Go and trips
+// users at every call site).
+var knownInterfaceTypes = map[string]bool{}
+
 // emitTypes renders types.gen.go.
 func (e *emitter) emitTypes() error {
 	t, err := template.New("types").Funcs(funcs(e.enums)).Parse(typesTmpl)
@@ -477,7 +486,7 @@ func goType(tr spec.TypeRef, optional bool) string {
 		//    multipart helpers (fileCheck, multipartFileEntry) call
 		//    f.IsLocalUpload() and dereference Reader, both of which
 		//    expect a pointer receiver.
-		if _, isUnion := knownDiscriminators[tr.Name]; isUnion {
+		if knownInterfaceTypes[tr.Name] {
 			// Interface type — never add *.
 			return tr.Name
 		}