refactor: Rework C# codegen into Handlebars templates

Replace the verbatim port of the nextlove C# AST serializer with the
Handlebars-template + context architecture used by the python and
javascript-http codegen. Serialization now lives in .hbs templates driven by
a durable data model and model builder; TEMPORARY banners are reserved for the
raw-OpenAPI parsing helpers only, matching the python migration.

- Delete the verbatim AST framework (codegen.ts), the serializer
  (dataclass.ts), and the main loop (generate-csharp-sdk.ts).
- Add durable class-model.ts (data interfaces) and build-model.ts (the
  traversal that produces the data model, preserving every output quirk:
  group-name reversal, is_void logic, DataContract naming, member order,
  inline enums, oneOf/JsonSubtypes unions, DeviceProperties forceNullable,
  errors/warnings message override, reserved keywords, Object vs object).
- Add .hbs templates: model.hbs, api.hbs and partials (model-class, enum-def,
  data-member, oneof-union, route-methods, tostring); load partials via
  getHandlebarsPartials; add csParams/csNamedArgs/eq helpers.
- Stop vendoring static .cs into codegen: the Client/* runtime, the two static
  Model helpers, and the .sln are normal committed package source under
  output/csharp/, not codegen assets (remove templates/fs/** and
  populate-fs.ts). The generator now emits only schema-derived Api/*.cs and
  Model/*.cs, mirroring how the python codegen only writes seam/routes/.
- Remove dead endpoint-rules.ts.

Generated output verified byte-identical after csharpier 0.29.2; npm run lint,
typecheck, and format pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018oCdmrhyAEX8e9sAFRXZAF

Author: claude

codegen/layouts/api.hbs

@@ -0,0 +1,52 @@
+using System.Runtime.Serialization;
+using System.Text;
+using JsonSubTypes;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+using Newtonsoft.Json.Linq;
+using Seam.Client;
+using Seam.Model;
+
+namespace Seam.Api
+{
+public class {{className}}
+{
+private ISeamClient _seam;
+
+public {{className}}(ISeamClient seam)
+{
+_seam = seam;
+}
+{{#each routes}}
+
+{{> model-class request}}
+{{#each requestSiblings}}
+
+{{> model-class this}}
+{{/each}}
+{{#if response}}
+
+{{> model-class response}}
+{{/if}}
+{{#each responseSiblings}}
+
+{{> model-class this}}
+{{/each}}
+
+{{> route-methods this}}
+{{/each}}
+}
+}
+
+namespace Seam.Client
+{
+public partial class SeamClient
+{
+public Api.{{className}} {{className}} => new(this);
+}
+
+public partial interface ISeamClient
+{
+public Api.{{className}} {{className}} { get; }
+}
+}

codegen/layouts/model.hbs

@@ -0,0 +1,19 @@
+using System.Runtime.Serialization;
+using System.Text;
+using JsonSubTypes;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+using Newtonsoft.Json.Linq;
+using Seam.Model;
+
+namespace Seam.Model
+{
+{{#each decls}}
+{{#if (eq kind "union")}}
+{{> oneof-union this}}
+{{else}}
+{{> model-class this}}
+{{/if}}
+
+{{/each}}
+}

codegen/layouts/partials/data-member.hbs

@@ -0,0 +1,2 @@
+[DataMember(Name = "{{snakeName}}", IsRequired = {{isRequired}}, EmitDefaultValue = false)]
+public {{#if isOverride}}override {{/if}}{{type}} {{pascalName}} {{#if getOnly}}{ get; }{{else}}{ get; set; }{{/if}}{{#if initializer}} = {{initializer}};{{/if}}

codegen/layouts/partials/enum-def.hbs

@@ -0,0 +1,13 @@
+{{#if isString}}
+[JsonConverter(typeof(SafeStringEnumConverter))]
+{{/if}}
+public enum {{name}}
+{
+{{#each members}}
+[EnumMember(Value = {{#if ../isString}}"{{value}}"{{else}}{{value}}{{/if}})]
+{{identifier}} = {{assign}},
+{{#unless @last}}
+
+{{/unless}}
+{{/each}}
+}

codegen/layouts/partials/model-class.hbs

@@ -0,0 +1,29 @@
+[DataContract(Name = "{{dataContractName}}")]
+public class {{className}}{{#if baseClass}} : {{baseClass}}{{/if}}
+{
+[JsonConstructorAttribute]
+{{#if properties.length}}protected{{else}}public{{/if}} {{className}}() { }
+{{#if properties.length}}
+
+public {{className}}({{csParams properties}})
+{
+{{#each properties}}
+{{pascalName}} = {{camelName}};
+{{/each}}
+}
+{{/if}}
+{{#each nested}}
+
+{{#if enum}}
+{{> enum-def enum}}
+{{else}}
+{{> oneof-union union}}
+{{/if}}
+{{/each}}
+{{#each properties}}
+
+{{> data-member this}}
+{{/each}}
+
+{{> tostring}}
+}

codegen/layouts/partials/oneof-union.hbs

@@ -0,0 +1,17 @@
+[JsonConverter(typeof(JsonSubtypes), "{{discriminatorSnake}}")]
+[JsonSubtypes.FallBackSubType(typeof({{unrecognizedTypeName}}))]
+{{#each knownSubTypes}}
+[JsonSubtypes.KnownSubType(typeof({{typeName}}), "{{value}}")]
+{{/each}}
+public abstract class {{className}}
+{
+{{#each abstractProps}}
+public abstract {{type}} {{pascalName}} {{#if getOnly}}{ get; }{{else}}{ get; set; }{{/if}}
+
+{{/each}}
+public abstract override string ToString();
+}
+{{#each subclasses}}
+
+{{> model-class this}}
+{{/each}}

codegen/layouts/partials/route-methods.hbs

@@ -0,0 +1,39 @@
+public {{#if isVoid}}void{{else}}{{returnType}}{{/if}} {{methodName}}({{methodName}}Request request)
+{
+var requestOptions = new RequestOptions();
+requestOptions.Data = request;
+{{#if isVoid}}
+_seam.Post<object>("{{path}}", requestOptions);
+{{else}}
+return _seam.Post<{{responseTypeArg}}>("{{path}}", requestOptions).Data.{{returnProp}};
+{{/if}}
+}
+
+public {{#if isVoid}}void{{else}}{{returnType}}{{/if}} {{methodName}}({{csParams params}})
+{
+{{#if isVoid}}
+{{methodName}}(new {{methodName}}Request({{csNamedArgs params}}));
+{{else}}
+return {{methodName}}(new {{methodName}}Request({{csNamedArgs params}}));
+{{/if}}
+}
+
+public async {{#if isVoid}}Task{{else}}Task<{{returnType}}>{{/if}} {{methodName}}Async({{methodName}}Request request)
+{
+var requestOptions = new RequestOptions();
+requestOptions.Data = request;
+{{#if isVoid}}
+await _seam.PostAsync<object>("{{path}}", requestOptions);
+{{else}}
+return (await _seam.PostAsync<{{responseTypeArg}}>("{{path}}", requestOptions)).Data.{{returnProp}};
+{{/if}}
+}
+
+public async {{#if isVoid}}Task{{else}}Task<{{returnType}}>{{/if}} {{methodName}}Async({{csParams params}})
+{
+{{#if isVoid}}
+await {{methodName}}Async(new {{methodName}}Request({{csNamedArgs params}}));
+{{else}}
+return (await {{methodName}}Async(new {{methodName}}Request({{csNamedArgs params}})));
+{{/if}}
+}

codegen/layouts/partials/tostring.hbs

@@ -0,0 +1,18 @@
+public override string ToString()
+{
+JsonSerializer jsonSerializer = JsonSerializer.CreateDefault(null);
+
+StringWriter stringWriter = new StringWriter(
+new StringBuilder(256),
+System.Globalization.CultureInfo.InvariantCulture
+);
+using (JsonTextWriter jsonTextWriter = new JsonTextWriter(stringWriter))
+{
+jsonTextWriter.IndentChar = ' ';
+jsonTextWriter.Indentation = 2;
+jsonTextWriter.Formatting = Formatting.Indented;
+jsonSerializer.Serialize(jsonTextWriter, this, null);
+}
+
+return stringWriter.ToString();
+}