fix(test): isolate tests from mock.module cache pollution

mock.module() in index-execute.test.ts permanently poisons Bun's
module cache — mock.restore() does not undo it in Bun 1.3.11.

Use cache-busted dynamic imports (../src/mod.ts?t=${Date.now()}) in
beforeEach for the three affected test files so each test gets a
fresh, unpoisoned module instance.

test/search/client.test.ts

@@ -1,8 +1,9 @@
-import { afterEach, describe, expect, test } from "bun:test";
+import { afterEach, beforeEach, describe, expect, test } from "bun:test";
 
-import { searchPerplexity } from "../../src/search/client.js";
 import { SearchError } from "../../src/search/types.js";
 
+let searchPerplexity: typeof import("../../src/search/client.js").searchPerplexity;
+
 function createSseResponse(events: Array<Record<string, unknown>>, status = 200): Response {
   const streamText = [
     ...events.map((event) => `data: ${JSON.stringify(event)}\n\n`),
@@ -18,6 +19,11 @@ function createSseResponse(events: Array<Record<string, unknown>>, status = 200)
 describe("searchPerplexity", () => {
   const originalFetch = globalThis.fetch;
 
+  beforeEach(async () => {
+    const mod = await import(`../../src/search/client.ts?t=${Date.now()}`);
+    searchPerplexity = mod.searchPerplexity;
+  });
+
   afterEach(() => {
     globalThis.fetch = originalFetch;
   });