fix: scope the System.Text.Json reference to netstandard2.0, name the failing member

Two issues from review:

- System.Text.Json was referenced unconditionally, pushing a dependency
  constraint onto net8.0+ consumers that already have it in-box. Now
  conditioned on netstandard2.0, which is the target that actually needs it.
  netstandard2.0 still resolves 9.0.9, so there is no NU1605 downgrade.
- Argument validation reported nameof(request) when it was request.Text that
  was empty, pointing callers at the wrong member.

Author: lukekim

Spice/Spice.csproj

@@ -56,12 +56,12 @@
     </ItemGroup>
 
     <!--
-        System.Text.Json arrives transitively via Apache.Arrow.Adbc, but search
-        serialization depends on it directly, so pin it explicitly. The version matches
-        what Apache.Arrow.Adbc already resolves to; netstandard2.0 needs the package
-        because, unlike net8.0+, it has no in-box System.Text.Json.
+        netstandard2.0 has no in-box System.Text.Json, so it needs the package. net8.0+
+        ship it in the framework, and referencing it there would push an unnecessary
+        dependency constraint onto consumers. The version matches what
+        Apache.Arrow.Adbc already resolves to, so netstandard2.0 sees no downgrade.
     -->
-    <ItemGroup>
+    <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
         <PackageReference Include="System.Text.Json" Version="9.0.9" />
     </ItemGroup>
 

Spice/src/Http/SpiceHttpClient.cs

@@ -160,11 +160,11 @@ public async Task<SearchResponse> SearchAsync(SearchRequest request, Cancellatio
 #if NET8_0_OR_GREATER
         ObjectDisposedException.ThrowIf(_disposed, this);
         ArgumentNullException.ThrowIfNull(request);
-        ArgumentException.ThrowIfNullOrWhiteSpace(request.Text, nameof(request));
+        ArgumentException.ThrowIfNullOrWhiteSpace(request.Text, $"{nameof(request)}.{nameof(request.Text)}");
 #else
         if (_disposed) throw new ObjectDisposedException(GetType().FullName);
         ThrowHelper.ThrowIfNull(request, nameof(request));
-        ThrowHelper.ThrowIfNullOrWhiteSpace(request.Text, nameof(request));
+        ThrowHelper.ThrowIfNullOrWhiteSpace(request.Text, $"{nameof(request)}.{nameof(request.Text)}");
 #endif
 
         var url = $"{_httpAddress}/v1/search";