fix(matrix): also pin test imports to matrix-js-sdk subpath

Two companion test files still used the bare matrix-js-sdk import while
the production code in event-helpers.ts had been migrated to the
matrix-js-sdk/lib/matrix.js subpath:

- extensions/matrix/src/matrix/sdk/event-helpers.test.ts (flagged by greptile P2)
- extensions/matrix/src/matrix/client/file-sync-store.test.ts (found via grep
  while addressing the P2)

After this commit, all matrix-js-sdk imports across extensions/matrix/src
go through the same subpath. Verified with:

  grep -rn 'from "matrix-js-sdk"' extensions/matrix/src/ | grep -v lib

returns no results.

Addresses greptile P2 finding on PR #68498.

extensions/matrix/src/matrix/client/file-sync-store.test.ts

@@ -1,7 +1,7 @@
 import fs from "node:fs";
 import os from "node:os";
 import path from "node:path";
-import type { ISyncResponse } from "matrix-js-sdk";
+import type { ISyncResponse } from "matrix-js-sdk/lib/matrix.js";
 import * as jsonStore from "openclaw/plugin-sdk/json-store";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { FileBackedMatrixSyncStore } from "./file-sync-store.js";

extensions/matrix/src/matrix/sdk/event-helpers.test.ts

@@ -1,4 +1,4 @@
-import type { MatrixEvent } from "matrix-js-sdk";
+import type { MatrixEvent } from "matrix-js-sdk/lib/matrix.js";
 import { describe, expect, it } from "vitest";
 import { buildHttpError, matrixEventToRaw, parseMxc } from "./event-helpers.js";