fix(flow-react): avoid Flow-React portal outlet removal conflicts

[platosha]

Some routing cases in hybrid Flow layout + React view applications could produce DOM tree conflicts from Flow server-side changes and React client-side portal removal happening simultaneously. This could throw DOM NotFoundError in the browser. This change introduces a dedicated DOM element for React portal outlet, which allows to avoid the error.

Fixes vaadin/hilla#3002

[github-actions]

Test Results

  890 files   - 269    890 suites   - 269   1h 4m 34s ⏱️ - 29m 52s
7 275 tests  - 304  6 800 ✅  -   723  48 💤  -  8  43 ❌ +43  384 🔥 +384 
7 291 runs   - 642  6 826 ✅  - 1 042  31 💤  - 34  43 ❌ +43  391 🔥 +391 

For more details on these failures and errors, see this check.

Results for commit 6ee4e01. ± Comparison against base commit 09439d3.

This pull request removes 304 tests.

com.vaadin.flow.custom.CustomRouteIT ‑ CustomRegistry_hasExpectedErrorHandlers[any_Chrome_]
com.vaadin.flow.custom.CustomRouteIT ‑ testCustomErrorView[any_Chrome_]
com.vaadin.flow.misc.ui.CompressedResourceIT ‑ compressedResourcesAreServedCompressed[any_Firefox_]
com.vaadin.flow.misc.ui.ExceptionLoggingIT ‑ productionModeExceptionViaExternalJsShouldLogException[any_Chrome_]
com.vaadin.flow.misc.ui.ExceptionLoggingIT ‑ productionModeExceptionViaServerJsShouldLogException[any_Chrome_]
com.vaadin.flow.misc.ui.JavascriptServedAsUtf8IT ‑ loadJavascriptWithUtf8[any_Chrome_]
com.vaadin.flow.misc.ui.MiscelaneousIT ‑ handlesIncorrectIconProperly[any_Chrome_]
com.vaadin.flow.misc.ui.MiscelaneousIT ‑ should_loadThemedComponent_fromLocal[any_Chrome_]
com.vaadin.flow.misc.ui.NpmThemedComponentIT ‑ importedClientSideComponentIsThemed[any_Chrome_]
com.vaadin.flow.misc.ui.PreserveOnRefreshCloseUIsIT ‑ preserveOnRefreshUiClosed_weakReferenceCleanedAfterGC[any_Chrome_]
…

♻️ This comment has been updated with latest results.

[caalador]

Still doesn't happen when manually running, but if I have the server open and run the hillaViewInFlowLayout test from vaadin-platform-react-hybrid-test it always fails with

Unexpected Application Error!
Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.
NotFoundError: Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.
    at removeChildFromContainer (http://localhost:8080/VAADIN/@fs/C:/Users/.../platform/vaadin-platform-react-hybrid-test/node_modules/.vite/deps/chunk-UJKGNB4G.js?v=3c974d7f:8509:23)
    ```

[sonarqubecloud]

Quality Gate passed

Issues
120 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud