feat(lotus-denoise): AGC off for ML tier + init/build leak hardening

AEC/AGC audit fix + two hardening items from the engine review.

- Add an `autoGainControl` capture param (UrlParams -> CallViewModel ->
  ConnectionFactory audioCaptureDefaults), mirroring echoCancellation/
  noiseSuppression. Defaults true (unchanged); the host sets it false only for
  the ML tier so the browser's auto gain control doesn't fight the in-source ML
  denoiser (pumping). Echo cancellation stays on. Tests cover the URL parse and
  the audioCaptureDefaults wiring.
- L1: init() now closes the owned AudioContext on a build failure (was orphaned;
  browsers cap live contexts, so repeated failures could exhaust them).
- L2: buildGraph() disposes its partially-built nodes on failure (disposeGraph
  previously only cleaned the prior graph).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/state/CallViewModel/remoteMembers/ECConnectionFactory.test.ts

@@ -53,14 +53,13 @@ beforeEach(() => {
 
 describe("ECConnectionFactory - Audio inputs options", () => {
   test.each([
-    { echo: true, noise: true },
-    { echo: true, noise: false },
-    { echo: false, noise: true },
-    { echo: false, noise: false },
+    { echo: true, noise: true, agc: true },
+    { echo: true, noise: false, agc: false },
+    { echo: false, noise: true, agc: false },
+    { echo: false, noise: false, agc: true },
   ])(
-    "it sets echoCancellation=$echo and noiseSuppression=$noise based on constructor parameters",
-    ({ echo, noise }) => {
-      // test("it sets echoCancellation and noiseSuppression based on constructor parameters", () => {
+    "it sets echoCancellation=$echo, noiseSuppression=$noise, autoGainControl=$agc based on constructor parameters",
+    ({ echo, noise, agc }) => {
       const RoomConstructor = vi.mocked(LivekitRoom);
 
       const ecConnectionFactory = new ECConnectionFactory(
@@ -76,6 +75,7 @@ describe("ECConnectionFactory - Audio inputs options", () => {
         undefined,
         echo,
         noise,
+        agc,
       );
       ecConnectionFactory.createConnection(
         testScope,
@@ -90,6 +90,7 @@ describe("ECConnectionFactory - Audio inputs options", () => {
           audioCaptureDefaults: expect.objectContaining({
             echoCancellation: echo,
             noiseSuppression: noise,
+            autoGainControl: agc,
           }),
         }),
       );