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.

api/methods.gen.go

@@ -2676,7 +2676,23 @@ type GetChatAdministratorsParams struct {
 //
 // Use this method to get a list of administrators in a chat. Returns an Array of ChatMember objects.
 func GetChatAdministrators(ctx context.Context, b *client.Bot, p *GetChatAdministratorsParams) ([]ChatMember, error) {
-	return client.Call[*GetChatAdministratorsParams, []ChatMember](ctx, b, "getChatAdministrators", p)
+	raw, err := client.CallRaw[*GetChatAdministratorsParams](ctx, b, "getChatAdministrators", p)
+	if err != nil {
+		return nil, err
+	}
+	var elems []json.RawMessage
+	if err := json.Unmarshal(raw, &elems); err != nil {
+		return nil, err
+	}
+	out := make([]ChatMember, 0, len(elems))
+	for _, e := range elems {
+		v, err := UnmarshalChatMember(e)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, v)
+	}
+	return out, nil
 }
 
 // GetChatMemberCountParams is the parameter set for GetChatMemberCount.