Simplify text completion generator boundary

ts/packages/flow/src/__tests__/text-completion-common.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "@effect/vitest";
+import type { LlmChunk } from "@trustgraph/base";
+import { providerRuntimeError, toAsyncGenerator } from "../model/text-completion/common.js";
+
+const emptyChunkIterator = (): AsyncIterable<LlmChunk> => ({
+  [Symbol.asyncIterator]: () => ({
+    next: () => Promise.resolve({ done: true, value: undefined }),
+  }),
+});
+
+describe("text completion common helpers", () => {
+  it("maps fallback generator throw failures into tagged provider errors", async () => {
+    const generator = toAsyncGenerator(
+      emptyChunkIterator(),
+      (error) => providerRuntimeError("test-provider", error),
+    );
+
+    await expect(generator.throw("provider failed")).rejects.toMatchObject({
+      _tag: "TextCompletionProviderError",
+      provider: "test-provider",
+      message: "provider failed",
+    });
+  });
+});