feat(searxng): configure endpoint via SEARXNG_URL env (#2499)

Add SEARXNG_URL runtime override for the built-in SearXNG web-search
provider, defaulting to http://localhost:8888/search. Enables Docker
and remote SearXNG deployments without changing existing behavior.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/searxng-provider-config.test.js

@@ -0,0 +1,30 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+const originalSearxngUrl = process.env.SEARXNG_URL;
+
+async function loadProvider(url) {
+  if (url === undefined) delete process.env.SEARXNG_URL;
+  else process.env.SEARXNG_URL = url;
+  vi.resetModules();
+  return (await import("../../open-sse/providers/registry/searxng.js")).default;
+}
+
+afterEach(() => {
+  if (originalSearxngUrl === undefined) delete process.env.SEARXNG_URL;
+  else process.env.SEARXNG_URL = originalSearxngUrl;
+  vi.resetModules();
+});
+
+describe("SearXNG provider configuration", () => {
+  it("uses SEARXNG_URL when the deployment config supplies one", async () => {
+    const provider = await loadProvider("http://searxng:8080/search");
+
+    expect(provider.searchConfig.baseUrl).toBe("http://searxng:8080/search");
+  });
+
+  it("preserves the loopback default when SEARXNG_URL is unset", async () => {
+    const provider = await loadProvider(undefined);
+
+    expect(provider.searchConfig.baseUrl).toBe("http://localhost:8888/search");
+  });
+});