fix(dotenv): encode []string as CSV instead of Go bracket notation

internal/encoding/dotenv/codec.go

@@ -32,7 +32,17 @@ func (Codec) Encode(v map[string]any) ([]byte, error) {
 	var buf bytes.Buffer
 
 	for _, key := range keys {
-		_, err := buf.WriteString(fmt.Sprintf("%v=%v\n", strings.ToUpper(key), flattened[key]))
+		var valStr string
+		switch v := flattened[key].(type) {
+		case []string:
+			// Encode string slices as comma-separated values.
+			// Using fmt's %v would produce "[a b]" which gotenv reads back
+			// as a single string instead of a slice.
+			valStr = strings.Join(v, ",")
+		default:
+			valStr = fmt.Sprintf("%v", v)
+		}
+		_, err := buf.WriteString(fmt.Sprintf("%v=%v\n", strings.ToUpper(key), valStr))
 		if err != nil {
 			return nil, err
 		}

internal/encoding/dotenv/codec_test.go

@@ -53,3 +53,22 @@ func TestCodec_Decode(t *testing.T) {
 		t.Logf("decoding failed as expected: %s", err)
 	})
 }
+
+// TestCodec_Encode_StringSlice verifies that []string values are encoded as
+// comma-separated values rather than Go's default bracket notation "[a b]".
+//
+// dotenv has no array type, so the de-facto convention is CSV (VAL=a,b,c).
+// Viper's defaultDecoderConfig uses stringToWeakSliceHookFunc(",") which
+// reconstructs []string from a comma-separated string during Unmarshal,
+// completing the round-trip.
+func TestCodec_Encode_StringSlice(t *testing.T) {
+	codec := Codec{}
+
+	b, err := codec.Encode(map[string]any{
+		"HOSTS": []string{"host1:10101", "host2:10101"},
+	})
+	require.NoError(t, err)
+
+	// Must be comma-separated, not bracket notation.
+	assert.Equal(t, "HOSTS=host1:10101,host2:10101\n", string(b))
+}