Problem

Fixes #743

When a custom API returns an expando EntityCollection whose entities contain nested Entity or EntityCollection attribute values, the serialization produced verbose .NET-style JSON instead of the flat WebAPI-style JSON:

Before (broken):

{
  "user": {
    "Attributes": [{"Key": "systemuser", "Value": "a81ca1d0-..."}],
    "LogicalName": null,
    "Id": "00000000-..."
  }
}

After (fixed):

{
  "user": {
    "@odata.type": "systemuser",
    "@odata.id": "a81ca1d0-...",
    "__DisplayName__": "John Doe"
  }
}

Root Cause

In ExecuteMessageNode.SerializeAttributeValues(), the else branch passed Entity and EntityCollection attribute values directly to JsonConvert.SerializeObject() which used default .NET reflection serialization, producing the Attributes[] / Entities[] verbose format.

Fix

Refactored SerializeAttributeValues into a BuildAttributeDictionary() helper that returns a Dictionary<string, object>, adding recursive handling for:

• Entity attribute values → nested JSON object via recursion into BuildAttributeDictionary
• EntityCollection attribute values → JSON array of recursively serialized entity dictionaries

All existing type handling (OptionSetValue, Money, EntityReference, @odata.type annotations) is preserved unchanged.

Changes

• MarkMpn.Sql4Cds.Engine/ExecutionPlan/ExecuteMessageNode.cs — Core fix: refactored SerializeAttributeValues with a recursive BuildAttributeDictionary helper
• MarkMpn.Sql4Cds.Engine.Tests/ExpandoMessageExecutor.cs — New fake executor returning entities with nested Entity attributes
• MarkMpn.Sql4Cds.Engine.Tests/StubMessageCache.cs — Added ExpandoMessage entry for testing
• MarkMpn.Sql4Cds.Engine.Tests/FakeXrmEasyTestsBase.cs — Registered ExpandoMessageExecutor
• MarkMpn.Sql4Cds.Engine.Tests/JsonFunctionTests.cs — New test verifying flat WebAPI-style JSON output
• MarkMpn.Sql4Cds.Engine.Tests/MarkMpn.Sql4Cds.Engine.Tests.csproj — Added new executor file to project