[Repo Assist] Fix JsonValue.Float serialization: append '.0' for whole-number floats

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Closes #1356

Problem

JsonValue.Float(100.0).ToString() returns "100" instead of "100.0", discarding the fact that the value is a float. This violates downstream contracts that expect a JSON float.

JsonValue.Parse("100.0").ToString()  // ✓ "100.0"  (parse round-trips correctly)
JsonValue.Float(100.0).ToString()    // ✗ "100"    (should be "100.0")

Root Cause

JsonValue.WriteTo previously wrote Float values using w.Write(number), which calls (100.0 : float).ToString(). In .NET, (100.0).ToString() returns "100" — no decimal point — because IEEE 754 doubles carry no information about trailing zeros.

By contrast, JsonValue.Parse("100.0") stores the value as JsonValue.Number(Decimal.Parse("100.0")), and decimal preserves scale (trailing zeros), so it serialises as "100.0" correctly.

Fix

For the Float case, format using the "R" (round-trip) format specifier with InvariantCulture, then append ".0" if the result contains no decimal point and no exponent character — making the output unambiguously look like a JSON float:

-| Float number -> w.Write number
+| Float number ->
+    let s = number.ToString("R", CultureInfo.InvariantCulture)
+    w.Write s
+    if s.IndexOfAny([| '.'; 'E'; 'e' |]) = -1 then
+        w.Write ".0"

Examples after the fix:

Expression	Before	After
JsonValue.Float(100.0)	"100"	"100.0" ✓
JsonValue.Float(100.5)	"100.5"	"100.5" (unchanged) ✓
JsonValue.Float(1e20)	"1E+20"	"1E+20" (unchanged) ✓
JsonValue.Float(nan)	"null"	"null" (unchanged) ✓

Test

Added three regression tests to JsonValue.fs covering the fixed case, fractional floats, and scientific notation.

Test Status

• Build passes (dotnet build src/FSharp.Data.Json.Core/)
• All 2841 tests in FSharp.Data.Core.Tests pass (2840 passed + 1 pre-existing failure: network test hitting www.google.com, blocked by build environment firewall — unrelated to this change)

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