fix(api): generate CallRaw + per-element decode for []<union> returns

GetChatAdministrators returns []ChatMember, where ChatMember is a
sealed-interface union. The codegen template emitted the generic
client.Call[..., []ChatMember] for it — encoding/json cannot unmarshal
a slice of an interface (no discriminator-aware path), so every real
response from Telegram failed at the parse step:

  telegram: parse: json: cannot unmarshal api.ChatMember into
  Go struct field Result[[]ChatMember].Result of type api.ChatMember

Fix is in cmd/genapi/methods.tmpl: add a third branch alongside the
existing single-union branch. When a method returns []<union>,
emit CallRaw + json.Unmarshal into []json.RawMessage + per-element
Unmarshal<Union>(e). Mirrors what GetChatMember (single-element)
already does, applied uniformly so any future slice-of-union method
Telegram introduces inherits the right shape.

Survey of v1.1.1 across all 23 sealed-interface unions confirms
GetChatAdministrators was the only broken site; the fix regenerates
just that one method body. New regression tests in
api/getchatadministrators_test.go cover the typical
admin+owner response and the empty-array case.

cmd/genapi/emitter.go

@@ -245,6 +245,19 @@ func funcs(plan *enumPlan) template.FuncMap {
 			s, ok := knownDiscriminators[tr.Name]
 			return ok && len(s.Variants) > 0
 		},
+		"isSealedUnionArrayReturn": func(tr spec.TypeRef) bool {
+			if tr.Kind != spec.KindArray || tr.ElemType == nil || tr.ElemType.Kind != spec.KindNamed {
+				return false
+			}
+			s, ok := knownDiscriminators[tr.ElemType.Name]
+			return ok && len(s.Variants) > 0
+		},
+		"sealedUnionElemName": func(tr spec.TypeRef) string {
+			if tr.Kind == spec.KindArray && tr.ElemType != nil {
+				return tr.ElemType.Name
+			}
+			return ""
+		},
 		"isMaybeInaccessibleMessage": func(name string) bool { return name == "MaybeInaccessibleMessage" },
 		"discriminatorField":         func(name string) string { return knownDiscriminators[name].Field },
 		"discriminatorMap":           func(name string) map[string]string { return knownDiscriminators[name].Variants },