pack: exclude test sources from npm package and assert packaging in tests

- add src/.npmignore to exclude src/*.test.ts and nested test files when packaging
- update src/package-manifest.test.ts to assert no .test.ts files are included in npm pack

src/.npmignore

@@ -0,0 +1,3 @@
+# Exclude test sources from package when src/ is included via "files"
+*.test.ts
+**/*.test.ts

src/package-manifest.test.ts

@@ -58,4 +58,7 @@ test("npm pack includes runtime package assets", () => {
   assert.ok(packedPaths.includes("package.json"));
   assert.ok(packedPaths.includes("index.ts"));
   assert.ok(packedPaths.includes("src/runtime.ts"));
+
+  // Ensure no test source files are included in npm pack
+  assert.ok(!packedPaths.some((p) => p.endsWith(".test.ts")));
 });