Fix JsonValue.Float serialization: append '.0' for whole-number floats

JsonValue.Float(100.0).ToString() previously returned "100" instead of
"100.0", discarding the fact that the value is a float. This causes issues
when downstream consumers expect a JSON float (not an integer).

Root cause: the write path used w.Write(number) for the Float DU case which
calls number.ToString() — and (100.0).ToString() returns "100" in .NET.

Fix: format the float using the "R" round-trip format and, if the result
contains no decimal point or exponent character, append ".0". This matches
the behaviour of JsonValue.Parse("100.0") which correctly serialises as
"100.0" (because decimal 100M with Scale=1 already has the trailing zero).

Closes #1356

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

Author: github-actions[bot]

src/FSharp.Data.Json.Core/JsonValue.fs

@@ -86,7 +86,13 @@ type JsonValue =
             | Boolean b -> w.Write(if b then "true" else "false")
             | Number number -> w.Write number
             | Float v when Double.IsInfinity v || Double.IsNaN v -> w.Write "null"
-            | Float number -> w.Write number
+            | Float number ->
+                let s = number.ToString("R", CultureInfo.InvariantCulture)
+                w.Write s
+                // Ensure the output looks like a float (has a decimal point or exponent),
+                // so that round-tripping through JSON preserves the Float type.
+                if s.IndexOfAny([| '.'; 'E'; 'e' |]) = -1 then
+                    w.Write ".0"
             | String s ->
                 w.Write "\""
                 JsonValue.JsonStringEncodeTo w s

tests/FSharp.Data.Core.Tests/JsonValue.fs

@@ -293,6 +293,20 @@ let ``Serializes special float value as null`` v =
     let json = JsonValue.Float v
     json.ToString(JsonSaveOptions.DisableFormatting) |> should equal "null"
 
+[<Test>]
+let ``Float value 100.0 serializes with decimal point`` () =
+    // Regression test for https://github.com/fsprojects/FSharp.Data/issues/1356
+    // JsonValue.Float(100.0) should serialize as "100.0", not "100"
+    JsonValue.Float(100.0).ToString(JsonSaveOptions.DisableFormatting) |> should equal "100.0"
+
+[<Test>]
+let ``Float value with fractional part serializes correctly`` () =
+    JsonValue.Float(100.5).ToString(JsonSaveOptions.DisableFormatting) |> should equal "100.5"
+
+[<Test>]
+let ``Float value in scientific notation serializes correctly`` () =
+    JsonValue.Float(1e20).ToString(JsonSaveOptions.DisableFormatting) |> should equal "1E+20"
+
 let normalize (str:string) =
   str.Replace("\r\n", "\n")
      .Replace("\r", "\n")