fix(csharp-codegen): FR-017 — emit TPH discriminator base as abstract (EF model build)

CI surfaced a real EF Core model-build failure on the C# TPH lanes:
  InvalidOperationException: The entity type 'Auth' has a discriminator
  property, but does not have a discriminator value configured.
A concrete base with HasDiscriminator + HasValue<Sub>(...) but no
HasValue<Base>(...) is invalid in EF Core; since every row is a concrete
subtype (no plain-base rows), the base must be abstract — then EF requires no
base discriminator value. Because EF validates the whole model on first use,
this single misconfig failed EVERY C# integration test (incl. non-TPH ones).

EntityGenerator now emits `public abstract class <Base>` for a TPH discriminator
base (TphPlanBuilder.IsTphDiscriminatorBase); subtypes/regular entities
unchanged. Regenerated the committed Generated/Auth.g.cs; updated TphCodegenTests.

Adds AppDbContextModelTests — a Docker-FREE guard that finalizes
`AppDbContext.Model` (EF builds the model offline, no DB connection) and
asserts the TPH hierarchy maps cleanly; this reproduces the exact CI error
without Testcontainers. Verified locally: model test 1/1, full Codegen.Tests
162/0/1 (incl. the fixture drift gate) — no Docker.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dmealing

server/csharp/MetaObjects.Codegen.Tests/TphCodegenTests.cs

@@ -95,7 +95,9 @@ public void Entity_base_carries_table_and_discriminator_property()
         var files = new EntityGenerator().Generate(Ctx(Load())).ToList();
         var auth = FileContent(files, "Auth.g.cs");
         Assert.Contains("[Table(\"auths\")]", auth);
-        Assert.Contains("public class Auth", auth);
+        // FR-017 TPH: the discriminator base is abstract (no plain-base rows; a
+        // concrete base with a discriminator + no base value fails EF model build).
+        Assert.Contains("public abstract class Auth", auth);
         // The discriminator field is a base property (the `type` enum -> AuthType).
         // Nullable because the `type` field is not @required and not in the PK; the
         // TS-owned `auths.type` column is likewise nullable TEXT.

server/csharp/MetaObjects.Codegen/Generators/EntityGenerator.cs

@@ -106,7 +106,13 @@ private EmittedFile EmitMappedClass(MetaObject entity, GenContext ctx)
         }
         if (!isProjection)
             sb.AppendLine($"[Table(\"{CSharpNaming.Table(entity)}\")]");
-        sb.AppendLine($"public class {className}");
+        // FR-017 TPH: the discriminator base is emitted `abstract` — every row is a
+        // concrete subtype (no plain-base rows exist), so EF Core must not require a
+        // base discriminator value. A concrete base with a discriminator but no
+        // HasValue<Base>(...) fails model validation ("has a discriminator property,
+        // but does not have a discriminator value configured").
+        var classKeyword = TphPlanBuilder.IsTphDiscriminatorBase(entity, ctx.Root) ? "abstract class" : "class";
+        sb.AppendLine($"public {classKeyword} {className}");
         sb.AppendLine("{");
 
         // Nested enum declarations (before the class properties, as C# enum members

server/csharp/MetaObjects.IntegrationTests/AppDbContextModelTests.cs

@@ -0,0 +1,39 @@
+// AppDbContextModelTests — Docker-FREE EF Core model-build gate.
+//
+// EF Core finalizes (validates) its model offline — no database connection is
+// opened just to build `context.Model`. So this test reproduces model-level
+// misconfiguration (e.g. a TPH discriminator base with no discriminator value)
+// WITHOUT Testcontainers. It is the fast guard for the class of failure that
+// otherwise only surfaces in the Docker integration suite.
+
+using MetaObjects.IntegrationTests.Generated;
+using Microsoft.EntityFrameworkCore;
+using Xunit;
+
+namespace MetaObjects.IntegrationTests;
+
+public class AppDbContextModelTests
+{
+    [Fact]
+    public void Model_builds_without_error()
+    {
+        // A dummy connection string is enough — model building never connects.
+        var options = new DbContextOptionsBuilder<AppDbContext>()
+            .UseNpgsql("Host=localhost;Database=unused")
+            .Options;
+        using var db = new AppDbContext(options);
+
+        // Forces EF model finalization. Before the FR-017 fix this threw
+        // InvalidOperationException: "The entity type 'Auth' has a discriminator
+        // property, but does not have a discriminator value configured."
+        var model = db.Model;
+
+        // The TPH hierarchy is mapped to one table with the discriminator + subtypes.
+        var auth = model.FindEntityType(typeof(Auth));
+        Assert.NotNull(auth);
+        Assert.NotNull(model.FindEntityType(typeof(BridgeAuth)));
+        Assert.NotNull(model.FindEntityType(typeof(CopayAuth)));
+        Assert.NotNull(model.FindEntityType(typeof(PriorAuthAuth)));
+        Assert.Equal("auths", auth!.GetTableName());
+    }
+}

server/csharp/MetaObjects.IntegrationTests/Generated/Auth.g.cs

@@ -9,7 +9,7 @@
 namespace MetaObjects.IntegrationTests.Generated;
 
 [Table("auths")]
-public class Auth
+public abstract class Auth
 {
     public enum AuthType { Bridge, Copay, PriorAuth }
     [Key]