Generalize browser automation hosting beyond the desktop app (#1878)

* feat(browser): generalize browser automation hosts

Rename the browser automation client capability to a host-neutral payload, advertise supported commands, and route tab-scoped commands by browserId affinity so future browser hosts can register without another protocol shape change.

* fix(browser): harden browser host registration

Reject unusable browser host capabilities, clean up pending requests when a host registration is replaced, and defer list_tabs affinity updates until every host in the aggregation succeeds.

* fix(browser): preserve host tab affinity across reconnects

Keep stranded tab ownership tied to the disconnected host so the same host can reclaim open tabs after reconnect, and tolerate future command names in browser_host capability payloads while retaining only known commands.

packages/app/src/browser-automation/handler.test.ts

@@ -396,7 +396,7 @@ describe("mountBrowserAutomationHandler", () => {
           ok: false,
           error: {
             code: "browser_unsupported",
-            message: "Desktop browser automation is not available in this app runtime.",
+            message: "Browser automation is not available in this app runtime.",
             retryable: false,
           },
         },
@@ -450,7 +450,7 @@ describe("mountBrowserAutomationHandler", () => {
           ok: false,
           error: {
             code: "browser_unsupported",
-            message: "Desktop browser automation is not implemented by this desktop build yet.",
+            message: "Browser automation is not implemented by this app build yet.",
             retryable: false,
           },
         },