feat(ai-router): test an endpoint before saving it, and re-point a live lane

Two gaps left by the AI Router feature: a profile could be saved without ever
being reachable, and a lane's router could only be chosen at creation.

**POST /api/ai-routers/test** answers the question saving cannot. It sends one
Anthropic-style message capped at a single output token to `<baseUrl>/v1/messages`
with a 15s timeout — the wire shape a claude lane's eight env keys point at, so
what is tested is what will actually run. A router that answers is a 200 here
whatever it replies: `{ ok, status, message }` carries the router's own words,
and `status` is null when nothing answered at all. Only bad input is a 4xx.

The stored key is only ever sent to the endpoint it was stored against. A blank
`apiKey` falls back to the key of the profile named by `name`, but only while
the submitted `baseUrl` still equals the saved one — so testing a retyped
endpoint needs a retyped key (`EBADKEY`). Without that rule this route would be
a way to make the server POST a saved API key to any URL a browser names.

Network failures report `error.cause`, not `error.message`: node's fetch reports
every transport failure as the same useless "fetch failed" and hides the part
the user can act on (ECONNREFUSED, ENOTFOUND, a TLS error) one level down.

**The lane card carries an AI Router select.** The Add-lane modal was the only
place a router was chosen, and a lane outlives its creation. Switching goes
through `POST /api/lanes/:id/router/apply` — never a lane PATCH — because the
DB column and the lane's own config on disk may only move together. A failed
apply puts the select back rather than leaving the card claiming a router the
lane did not get. Hidden when `native` is the only profile: a select with one
option is a control that cannot do anything.

Adds `Lane.router_profile` to the client type (the payload already carried it).

server/__tests__/ai-routers.test.js

@@ -188,3 +188,86 @@ describe("ai-routers — a corrupt file is skipped, never fatal", () => {
     assert.ok(!items.some((i) => i.name === "broken"));
   });
 });
+
+describe("ai-routers — testRouter", () => {
+  const realFetch = global.fetch;
+  after(() => {
+    global.fetch = realFetch;
+  });
+
+  /** Capture the one request testRouter makes, and answer it however asked. */
+  function stubFetch(answer) {
+    const calls = [];
+    global.fetch = async (url, init) => {
+      calls.push({ url, init });
+      if (answer instanceof Error) throw answer;
+      return answer;
+    };
+    return calls;
+  }
+
+  const ok = () => ({ ok: true, status: 200, statusText: "OK", text: async () => "{}" });
+
+  it("posts one capped message to the endpoint's /v1/messages", async () => {
+    const calls = stubFetch(ok());
+    const result = await routers.testRouter(nineRouter());
+    assert.equal(result.ok, true);
+    assert.equal(calls.length, 1);
+    assert.equal(calls[0].url, "http://localhost:20128/v1/v1/messages");
+    const body = JSON.parse(calls[0].init.body);
+    assert.equal(body.max_tokens, 1);
+    assert.equal(body.model, "Highest");
+    assert.equal(calls[0].init.headers["x-api-key"], "sk-secret-value");
+  });
+
+  it("reports a router that answered with an error rather than throwing", async () => {
+    stubFetch({ ok: false, status: 401, statusText: "Unauthorized", text: async () => "bad key" });
+    const result = await routers.testRouter(nineRouter());
+    assert.deepEqual(result, { ok: false, status: 401, message: "bad key" });
+  });
+
+  it("reports an endpoint that never answered", async () => {
+    // The shape node's own fetch throws: a useless outer message wrapping the
+    // one thing the user can act on.
+    stubFetch(
+      Object.assign(new Error("fetch failed"), {
+        cause: new Error("connect ECONNREFUSED 127.0.0.1:20199"),
+      })
+    );
+    const result = await routers.testRouter(nineRouter());
+    assert.equal(result.ok, false);
+    assert.equal(result.status, null);
+    assert.match(result.message, /ECONNREFUSED/);
+  });
+
+  it("falls back to the stored key for the endpoint it was stored against", async () => {
+    const created = routers.createRouter(nineRouter());
+    const calls = stubFetch(ok());
+    await routers.testRouter({
+      name: created.name,
+      baseUrl: "http://localhost:20128/v1",
+      model: "Highest",
+    });
+    assert.equal(calls[0].init.headers["x-api-key"], "sk-secret-value");
+  });
+
+  it("never sends the stored key to a different endpoint", async () => {
+    const created = routers.createRouter(nineRouter());
+    const calls = stubFetch(ok());
+    await assert.rejects(
+      routers.testRouter({
+        name: created.name,
+        baseUrl: "https://attacker.example/v1",
+        model: "Highest",
+      }),
+      { code: "EBADKEY" }
+    );
+    assert.equal(calls.length, 0);
+  });
+
+  it("refuses a base URL that is not http(s)", async () => {
+    await assert.rejects(routers.testRouter(nineRouter({ baseUrl: "file:///etc/passwd" })), {
+      code: "EBADURL",
+    });
+  });
+});