Remove dependency on Newtonsoft.Json from Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost

[Copilot]

Migrates the MSBuild.BuildHost RPC layer from Newtonsoft.Json to System.Text.Json.

Changes

• JsonSettings.cs: Replace JsonSerializerSettings with JsonSerializerOptions
  • CamelCasePropertyNamesContractResolver → JsonNamingPolicy.CamelCase
  • StringEscapeHandling.EscapeNonAscii → JavaScriptEncoder.Create(UnicodeRanges.BasicLatin)
• Request.cs / Response.cs: JToken → JsonElement
• RpcServer.cs / RpcClient.cs: JsonConvert → JsonSerializer
• Package references: Remove Newtonsoft.Json, add System.Text.Json for net472 only (BCL for .NET Core)

// Before
request.Parameters[i].ToObject(methodParameters[i].ParameterType)
JToken.FromObject(result)

// After
request.Parameters[i].Deserialize(methodParameters[i].ParameterType, JsonSettings.SingleLineSerializerOptions)
JsonSerializer.SerializeToElement(result, JsonSettings.SingleLineSerializerOptions)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove dependency on Newtonsoft.Json from Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost project</issue_title>
<issue_description>Use System.Text.Json instead.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove dependency on Newtonsoft.Json from Microsoft.CodeAnalysis.Workspaces.MSBuild.BuildHost project #81561

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[tmat]

@jasonmalinowski ptal