Restructure ODataMetadata to mirror XML Schema hierarchy

[Copilot]

ODataEntityContainer was renamed to ODataSchema but still only held EntityContainer-level data. The ODataMetadata root object held flat aggregated lists that didn't reflect the actual <Schema> → <EntityContainer> nesting in OData $metadata XML.

Structural changes

• ODataMetadata stripped to two fields only:

  public class ODataMetadata {
      public ODataVersion ODataVersion { get; set; }
      public List<ODataSchema> Schemas { get; set; } = [];
  }

• ODataSchema now owns everything declared within a <Schema> element:

  public class ODataSchema {
      public string Namespace { get; set; }       // <Schema Namespace="...">
      public string? ContainerName { get; set; }  // <EntityContainer Name="...">
      public List<ODataEntityType> EntityTypes { get; set; }
      public List<ODataEnumType> EnumTypes { get; set; }
      public List<ODataEntitySet> EntitySets { get; set; }
      public List<ODataSingleton> Singletons { get; set; }
      public List<ODataFunction> Functions { get; set; }
  }

• Helper methods (GetDerivedTypes, GetAllDerivedTypes, SetEntityPaths) moved from ODataMetadata to ODataSchema. SetEntityPaths accepts a cross-schema entity type list to support multi-schema V4 documents; internally uses a pre-built lookup dict (O(n) vs prior O(n²)).

Parser changes

• V4 parser: creates one ODataSchema per XML <Schema> element; EntitySets/Singletons/Functions are populated only on the schema that owns the <EntityContainer>.
• V1/V3 parser: creates a single ODataSchema from the one XML schema element.
• MarkEnumProperties updated in both parsers to operate per-schema or across all schemas as appropriate.

Consumer updates

• ClientGenerator, ClientTemp, ClientHelperTemplate, MetadataExtensions: iterate metadata.Schemas instead of flat metadata.EntityTypes / metadata.Functions etc.
• Blazor doc components (GenerateClient.razor, ODataExplorer.razor): use SelectMany across schemas for aggregated display.
• MetadataTests: single-schema tests use metadata.Schemas[0].X; complex multi-schema V4 tests use per-schema access or metadata.Schemas.SelectMany(s => s.X).

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[joadan]

to match the XML structure we should move types, enums etc to the new ODataSchema