Add matrix_rtc_mode config option (#4014)

* Move MatrixRTCMode enum from settings.ts to ConfigOptions.ts

* Add matrix_rtc_mode config option

* add matrix_rtc_mode to config.sample.json

* Update src/settings/DeveloperSettingsTab.tsx

Co-authored-by: Johannes Marbach <n0-0ne+github@mailbox.org>

* Update src/settings/DeveloperSettingsTab.test.tsx

Co-authored-by: Johannes Marbach <n0-0ne+github@mailbox.org>

* reviewer comments

---------

Co-authored-by: Johannes Marbach <n0-0ne+github@mailbox.org>

src/config/Config.test.ts

@@ -0,0 +1,54 @@
+/*
+Copyright 2026 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { describe, expect, it, vi, afterEach } from "vitest";
+import { logger } from "matrix-js-sdk/lib/logger";
+
+import { validateConfig } from "./Config";
+import { MatrixRTCMode } from "./ConfigOptions";
+
+describe("validateConfig", () => {
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("passes through a missing matrix_rtc_mode unchanged", () => {
+    const result = validateConfig({});
+    expect(result.matrix_rtc_mode).toBeUndefined();
+  });
+
+  it.each(Object.values(MatrixRTCMode))(
+    "keeps a valid matrix_rtc_mode value (%s)",
+    (mode) => {
+      const warnSpy = vi.spyOn(logger, "warn").mockImplementation(() => {});
+      const result = validateConfig({ matrix_rtc_mode: mode });
+      expect(result.matrix_rtc_mode).toBe(mode);
+      expect(warnSpy).not.toHaveBeenCalled();
+    },
+  );
+
+  it("drops an invalid matrix_rtc_mode value and warns", () => {
+    const warnSpy = vi.spyOn(logger, "warn").mockImplementation(() => {});
+    const result = validateConfig({
+      // Intentionally bypass the type to simulate bad JSON.
+      matrix_rtc_mode: "nonsense" as unknown as MatrixRTCMode,
+    });
+    expect(result.matrix_rtc_mode).toBeUndefined();
+    expect(warnSpy).toHaveBeenCalledTimes(1);
+    expect(warnSpy.mock.calls[0][0]).toContain("nonsense");
+  });
+
+  it("does not touch unrelated fields when dropping an invalid mode", () => {
+    vi.spyOn(logger, "warn").mockImplementation(() => {});
+    const result = validateConfig({
+      matrix_rtc_mode: "nope" as unknown as MatrixRTCMode,
+      ssla: "https://example.invalid/ssla",
+    });
+    expect(result.matrix_rtc_mode).toBeUndefined();
+    expect(result.ssla).toBe("https://example.invalid/ssla");
+  });
+});