fix(ts-sdk): make natural an optional peer so pg is not installed transitively

natural@8 hard-depends on pg ^8.18.0, mongoose, redis and memjs. As a
required peer it was auto-installed for every mem0ai user, so pg stayed in
the tree even after pg itself became optional. lemmatizeForBm25 already
falls back to a built-in stemmer when natural is missing.

mem0-ts/package.json

@@ -263,6 +263,9 @@
     },
     "@types/pg": {
       "optional": true
+    },
+    "natural": {
+      "optional": true
     }
   },
   "engines": {

mem0-ts/src/oss/tests/missing-optional-peers.test.ts

@@ -2,7 +2,11 @@ jest.mock("pg", () => {
   throw new Error("Cannot find module 'pg'");
 });
 
-describe("pg is an optional peer", () => {
+jest.mock("natural", () => {
+  throw new Error("Cannot find module 'natural'");
+});
+
+describe("mem0ai/oss without pg or natural installed", () => {
   beforeEach(() => {
     jest.spyOn(console, "error").mockImplementation(() => {});
   });
@@ -11,7 +15,7 @@ describe("pg is an optional peer", () => {
     jest.restoreAllMocks();
   });
 
-  test("mem0ai/oss loads without pg installed", async () => {
+  test("mem0ai/oss loads", async () => {
     await expect(import("../src")).resolves.toHaveProperty("Memory");
   });
 
@@ -26,4 +30,10 @@ describe("pg is an optional peer", () => {
       "The 'pg' package is required to use the PGVector vector store. Install it with: npm install pg",
     );
   });
+
+  test("BM25 lemmatization falls back to the built-in stemmer", async () => {
+    const { lemmatizeForBm25 } = await import("../src/utils/lemmatization");
+
+    expect(lemmatizeForBm25("The dogs were running")).toBe("dog runn running");
+  });
 });