fix(js/ts): fix G/g format specifier corrupting exponential notation when trimming trailing zeros

The trimEnd() call applied to the full toPrecision() result was
incorrectly removing trailing zeros from the exponent part of numbers
in exponential notation. For example, (1e-10).ToString("G17") would
produce "1.0000000000000000e-1" instead of "1E-10" because trimEnd
matched the "0" in "-10".

Fix: split on the "e" separator, trim only the mantissa, then
recombine with the exponent. Uppercase "G" now also correctly produces
uppercase "E" in the exponent per .NET semantics.

Closes #2654

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: fable-repo-assist[bot]

src/fable-library-ts/String.ts

@@ -392,8 +392,15 @@ export function format(str: string | object, ...args: any[]) {
           break;
         case "g": case "G":
           rep = precision != null ? toPrecision(rep, precision) : toPrecision(rep);
-          // TODO: Check why some numbers are formatted with decimal part
-          rep = trimEnd(trimEnd(rep, "0"), ".");
+          // Handle exponential notation: only trim trailing zeros from mantissa, not from exponent
+          const eIdx = rep.indexOf("e");
+          if (eIdx >= 0) {
+            const mantissa = trimEnd(trimEnd(rep.slice(0, eIdx), "0"), ".");
+            const exponent = rep.slice(eIdx);
+            rep = mantissa + (format === "G" ? exponent.toUpperCase() : exponent);
+          } else {
+            rep = trimEnd(trimEnd(rep, "0"), ".");
+          }
           break;
         case "n": case "N":
           precision = precision != null ? precision : 2;

tests/Js/Main/StringTests.fs

@@ -511,6 +511,14 @@ let tests = testList "Strings" [
         String.Format(CultureInfo.InvariantCulture, "{0:P2}", 0.1234) |> equal "12.34 %"
         String.Format(CultureInfo.InvariantCulture, "{0:C2}", 1000) |> equal "¤1,000.00"
 
+    testCase "G format specifier trims trailing zeros without corrupting exponential notation" <| fun () -> // Fix #2654
+        (123.45).ToString("G17", CultureInfo.InvariantCulture) |> equal "123.45"
+        (10000.0).ToString("G17", CultureInfo.InvariantCulture) |> equal "10000"
+        (0.000222).ToString("G17", CultureInfo.InvariantCulture) |> equal "0.000222"
+        (1e-10).ToString("G17", CultureInfo.InvariantCulture) |> equal "1E-10"
+        (1e-10).ToString("g17", CultureInfo.InvariantCulture) |> equal "1e-10"
+        (1.5e10).ToString("G17", CultureInfo.InvariantCulture) |> equal "15000000000"
+
     testCase "ToString formatted works with decimals" <| fun () -> // See #2276
         let decimal = 78.6M
         decimal.ToString("0.000").Replace(",", ".") |> equal "78.600"