feat(webapp): inherit region on preview branches + test resolver

Preview branches copy the parent env's defaultWorkerGroupId. Adds a unit
test for the env -> project -> global fallback order.

apps/webapp/app/services/upsertBranch.server.ts

@@ -119,6 +119,8 @@ export class UpsertBranchService {
           pkApiKey,
           shortcode,
           maximumConcurrencyLimit: parentEnvironment.maximumConcurrencyLimit,
+          // Inherit the region from the parent preview environment.
+          defaultWorkerGroupId: parentEnvironment.defaultWorkerGroupId,
           organization: {
             connect: {
               id: parentEnvironment.organization.id,

apps/webapp/test/regionAccess.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from "vitest";
+import { resolveEffectiveDefaultWorkerGroupId } from "~/v3/regionAccess.server";
+
+describe("resolveEffectiveDefaultWorkerGroupId", () => {
+  it("prefers the environment default", () => {
+    expect(
+      resolveEffectiveDefaultWorkerGroupId({
+        environmentDefaultWorkerGroupId: "env",
+        projectDefaultWorkerGroupId: "project",
+        globalDefaultWorkerGroupId: "global",
+      })
+    ).toBe("env");
+  });
+
+  it("falls back to the project default when the environment has none", () => {
+    expect(
+      resolveEffectiveDefaultWorkerGroupId({
+        environmentDefaultWorkerGroupId: null,
+        projectDefaultWorkerGroupId: "project",
+        globalDefaultWorkerGroupId: "global",
+      })
+    ).toBe("project");
+  });
+
+  it("falls back to the global default when env and project have none", () => {
+    expect(
+      resolveEffectiveDefaultWorkerGroupId({
+        environmentDefaultWorkerGroupId: null,
+        projectDefaultWorkerGroupId: null,
+        globalDefaultWorkerGroupId: "global",
+      })
+    ).toBe("global");
+  });
+
+  it("returns undefined when nothing is set", () => {
+    expect(
+      resolveEffectiveDefaultWorkerGroupId({
+        environmentDefaultWorkerGroupId: null,
+        projectDefaultWorkerGroupId: null,
+        globalDefaultWorkerGroupId: null,
+      })
+    ).toBeUndefined();
+  });
+});