refactor(media): normalize inbound MediaType/MediaTypes defaults (#16233)

* refactor(media): normalize inbound media type defaults

* test(browser): fix Windows path expectation in file chooser hook

src/browser/server.agent-contract-form-layout-act-commands.test.ts

@@ -421,7 +421,8 @@ describe("browser control server", () => {
     expect(pwMocks.armFileUploadViaPlaywright).toHaveBeenCalledWith({
       cdpUrl: cdpBaseUrl,
       targetId: "abcd1234",
-      paths: [path.join(DEFAULT_UPLOAD_DIR, "a.txt")],
+      // The server resolves paths (which adds a drive letter on Windows for `\\tmp\\...` style roots).
+      paths: [path.resolve(DEFAULT_UPLOAD_DIR, "a.txt")],
       timeoutMs: 1234,
     });