Add unit tests for DoubleConversions::toString

Summary:
Add comprehensive unit tests for the `toString(double, char)` function in `DoubleConversions.cpp`. The tests cover:
- Zero and negative zero
- Positive and negative integers
- Fractional values
- Very small and very large values
- Scientific notation for values below 1e-6
- Infinity and NaN serialization as "0"
- Suffix character appending (including null suffix)
- Suffix behavior with special values (infinity, NaN)

Changelog: [Internal]

Differential Revision: D96625901

Author: christophpurrer

packages/react-native/ReactCommon/react/renderer/graphics/tests/DoubleConversionsTest.cpp

@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <react/renderer/graphics/DoubleConversions.h>
+
+#include <gtest/gtest.h>
+#include <cmath>
+#include <limits>
+
+using namespace facebook::react;
+
+TEST(DoubleConversionsTest, zeroValue) {
+  EXPECT_EQ(toString(0.0, '\0'), "0");
+}
+
+TEST(DoubleConversionsTest, negativeZero) {
+  EXPECT_EQ(toString(-0.0, '\0'), "-0");
+}
+
+TEST(DoubleConversionsTest, positiveIntegers) {
+  EXPECT_EQ(toString(1.0, '\0'), "1");
+  EXPECT_EQ(toString(42.0, '\0'), "42");
+  EXPECT_EQ(toString(1000.0, '\0'), "1000");
+}
+
+TEST(DoubleConversionsTest, negativeIntegers) {
+  EXPECT_EQ(toString(-1.0, '\0'), "-1");
+  EXPECT_EQ(toString(-42.0, '\0'), "-42");
+}
+
+TEST(DoubleConversionsTest, fractionalValues) {
+  EXPECT_EQ(toString(3.14, '\0'), "3.14");
+  EXPECT_EQ(toString(0.5, '\0'), "0.5");
+  EXPECT_EQ(toString(-2.75, '\0'), "-2.75");
+}
+
+TEST(DoubleConversionsTest, verySmallValues) {
+  EXPECT_EQ(toString(0.000001, '\0'), "0.000001");
+}
+
+TEST(DoubleConversionsTest, veryLargeValues) {
+  EXPECT_EQ(toString(1e20, '\0'), "100000000000000000000");
+}
+
+TEST(DoubleConversionsTest, scientificNotation) {
+  // Values below 1e-6 should use scientific notation
+  auto result = toString(1e-7, '\0');
+  EXPECT_NE(result.find('E'), std::string::npos);
+}
+
+TEST(DoubleConversionsTest, infinitySerializedAsZero) {
+  EXPECT_EQ(toString(std::numeric_limits<double>::infinity(), '\0'), "0");
+  EXPECT_EQ(toString(-std::numeric_limits<double>::infinity(), '\0'), "0");
+}
+
+TEST(DoubleConversionsTest, nanSerializedAsZero) {
+  EXPECT_EQ(toString(std::numeric_limits<double>::quiet_NaN(), '\0'), "0");
+}
+
+TEST(DoubleConversionsTest, suffixAppended) {
+  EXPECT_EQ(toString(3.14, '%'), "3.14%");
+  EXPECT_EQ(toString(100.0, 'x'), "100x");
+}
+
+TEST(DoubleConversionsTest, nullSuffixNotAppended) {
+  auto result = toString(42.0, '\0');
+  EXPECT_EQ(result, "42");
+  EXPECT_EQ(result.back(), '2');
+}
+
+TEST(DoubleConversionsTest, suffixWithSpecialValues) {
+  EXPECT_EQ(toString(std::numeric_limits<double>::infinity(), '%'), "0%");
+  EXPECT_EQ(toString(std::numeric_limits<double>::quiet_NaN(), 'x'), "0x");
+}