Add native LLM core foundation (#24712)

packages/llm/test/lib/sse.ts

@@ -0,0 +1,17 @@
+/**
+ * Helpers for building deterministic SSE bodies in tests.
+ *
+ * Inline template-literal SSE strings are hard to write and review when chunks
+ * contain JSON; this helper accepts plain values and serializes them, so test
+ * authors only think about the chunk shapes, not the wire format.
+ */
+export const sseEvents = (...chunks: ReadonlyArray<unknown>): string =>
+  `${chunks.map(formatChunk).join("")}data: [DONE]\n\n`
+
+const formatChunk = (chunk: unknown) => `data: ${typeof chunk === "string" ? chunk : JSON.stringify(chunk)}\n\n`
+
+/**
+ * Build an SSE body from already-serialized strings (used when the chunk shape
+ * itself is part of what's being tested, e.g. malformed chunks).
+ */
+export const sseRaw = (...lines: ReadonlyArray<string>): string => lines.map((line) => `${line}\n\n`).join("")