fix(typespec-ts): preserve optional api-version on client context

The rewritten client-context renderer was treating every api-version parameter as required based on isApiVersion alone, even when the adapter had already recorded a client default and left the parameter optional. That changed generated *Context interfaces from apiVersion?: string to apiVersion: string for defaulted clients.

Make requiredness follow the adapted TSClientParameter metadata instead of the api-version marker, and lock it in with a modular unit test that renders the new pipeline's client context for a versioned client with a defaulted api-version.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: maorleger

packages/typespec-ts/src/codegen/clients.ts

@@ -52,12 +52,7 @@ export function emitClientContext(
 
   // ── Client interface ──
   const requiredProperties = client.parameters
-    .filter(
-      (p) =>
-        !p.isEndpoint &&
-        !p.isCredential &&
-        (p.isApiVersion || (p.required && !p.hasDefaultValue))
-    )
+    .filter((p) => !p.isEndpoint && !p.isCredential && p.required)
     .map((p) => ({
       name: p.name,
       type: p.type,
@@ -331,7 +326,7 @@ function emitReturnStatement(
       !p.isEndpoint &&
       !p.isCredential &&
       p.name !== "options" &&
-      (p.isApiVersion || (p.required && !p.hasDefaultValue))
+      p.required
   );
 
   const requiredParamNames = new Set(

packages/typespec-ts/test/modularUnit/adapter.spec.ts

@@ -1,5 +1,8 @@
 import { describe, expect, it } from "vitest";
+import { Project } from "ts-morph";
 
+import { emitClientContext } from "../../src/codegen/clients.js";
+import { provideBinder } from "../../src/framework/hooks/binder.js";
 import { renameClientName } from "../../src/index.js";
 import { transformModularEmitterOptions } from "../../src/modular/buildModularOptions.js";
 import {
@@ -373,6 +376,44 @@ describe("tcgc adapter", () => {
     );
   });
 
+  it("keeps client-default api-version optional on emitted context interfaces", async () => {
+    const model = await adaptCodeModelFromTypeSpec(
+      buildServiceTypeSpec(
+        `
+          enum Versions {
+            v2026_05_15: "2026-05-15"
+          }
+
+          model ApiVersionParameter {
+            @query
+            "api-version": string;
+          }
+
+          @route("/widgets")
+          op ping(...ApiVersionParameter): void;
+        `,
+        `
+          @versioned(Versions)
+          @server("{endpoint}/widgets", "Widgets", {
+            endpoint: url
+          })
+        `
+      )
+    );
+
+    const client = model.clients[0]!;
+    const apiVersion = client.parameters.find((parameter) => parameter.isApiVersion);
+    expect(apiVersion).toMatchObject({ required: false, hasDefaultValue: true });
+
+    const project = new Project({ useInMemoryFileSystem: true });
+    const binder = provideBinder(project);
+    const file = emitClientContext(project, client, model.settings);
+    binder.resolveAllReferences("");
+
+    expect(file?.getFullText()).toContain("apiVersion?: string;");
+    expect(file?.getFullText()).not.toContain("apiVersion: string;");
+  });
+
   it("groups nested operations when operation groups are enabled", async () => {
     const client = await adaptFirstClientFromTypeSpec(
       buildServiceTypeSpec(`