Round debug float output to 4 decimal places (#55879)

Summary:
Pull Request resolved: #55879

Float values in debug layout metrics output (used by Fantom test assertions) are
subject to floating-point precision differences across platforms and compilers
(e.g., `66.6667` vs `66.66666412353516`), causing flaky test comparisons.

This changes `toString(const double&)` in `DebugStringConvertible.cpp` to format
floats with 4 decimal places using `snprintf("%.4f")` and strip trailing zeros,
replacing `double-conversion`'s `ToShortest()`. This is debug-only code (guarded
by `#if RN_DEBUG_STRING_CONVERTIBLE`) so the precision change is safe for all
consumers.

The `double-conversion` dependency is also removed from the BUCK file since it is
no longer used.

Output examples:
- `0.0` → `"0"`
- `100.0` → `"100"`
- `66.66666412353516` → `"66.6667"`
- `1.5` → `"1.5"`
- `33.333333` → `"33.3333"`

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D95041966

Author: NickGerleman

packages/react-native/ReactCommon/react/renderer/debug/DebugStringConvertible.cpp

@@ -7,11 +7,9 @@
 
 #include "DebugStringConvertible.h"
 
-#include <array>
-#include <cinttypes>
-#include <cstdio>
-
-#include <double-conversion/double-conversion.h>
+#include <cstdint>
+#include <iomanip>
+#include <sstream>
 
 namespace facebook::react {
 
@@ -129,33 +127,30 @@ SharedDebugStringConvertibleList DebugStringConvertible::getDebugProps() const {
  * `toString`-family implementation.
  */
 std::string toString(const double& value) {
-  // Format taken from folly's toString
-  static double_conversion::DoubleToStringConverter conv(
-      0,
-      "Infinity",
-      "NaN",
-      'E',
-      -6, // detail::kConvMaxDecimalInShortestLow,
-      21, // detail::kConvMaxDecimalInShortestHigh,
-      6, // max leading padding zeros
-      1); // max trailing padding zeros
-  std::array<char, 256> buffer{};
-  double_conversion::StringBuilder builder(buffer.data(), buffer.size());
-  conv.ToShortest(value, &builder);
-  return builder.Finalize();
+  std::ostringstream stream;
+  stream << std::fixed << std::setprecision(4) << value;
+  std::string result = stream.str();
+
+  // Strip trailing zeros and unnecessary decimal point
+  if (auto dotPos = result.find('.'); dotPos != std::string::npos) {
+    auto lastNonZero = result.find_last_not_of('0');
+    if (lastNonZero == dotPos) {
+      result.erase(dotPos);
+    } else {
+      result.erase(lastNonZero + 1);
+    }
+  }
+  return result;
 }
 
 std::string toString(const void* value) {
   if (value == nullptr) {
     return "null";
   }
-  std::array<char, 20> buffer{};
-  std::snprintf(
-      buffer.data(),
-      buffer.size(),
-      "0x%" PRIXPTR,
-      reinterpret_cast<uintptr_t>(value));
-  return buffer.data();
+  std::ostringstream stream;
+  stream << "0x" << std::uppercase << std::hex
+         << reinterpret_cast<uintptr_t>(value);
+  return stream.str();
 }
 
 #endif