Change priority of re-execution handling and allow router to stream NotFound contents

[ilonatommy]

Contributes to #62153.

Description

• Priority of re-execution middleware and rendering NotFound contents.

Old	Current
We used to detect the status code in the rendering method to avoid flushing the response and hand-over the processing to re-execution middleware. We no longer do that and we removed nulling-out the response content type because it was done only to force re-execution middleware to process the request further.	If re-execution middleware is set up together with the default Router, then in all the cases we should render NotFoundPage contents and if page is not available, then we can render NotFound fragment if streaming has not started.

• In case we are streaming, we should allow the client to use enhanced navigation to render NotFound contents.

Old	Current
Inject JS-redirection into the page, which changed the url and introduced inconsistent behavior in comparison to non-streaming scenarios.	If enhanced navigation is available, we rely on it to render the NotFound contents but prevent the url change. This will happen on JS-initiated navigations. If enhanced navigation is off or the navigation is initiated by the browser (e.g. mistype in the url bar) then we redirect to NotFound, which includes url change.

• Customized NotFound url when streaming:

Old	Current
We used to redirect to fixed "not-found".	If Router has NotFoundPage type passed, it's getting rendered, otherwise we rely on the url passed to re-execution middleware. NotFound fragment cannot be rendered this way.

• Adding Components.Shared.csproj was needed because we want the same components to be tested in global interactivity application (InteractivityTests.cs that is running Components.WasmMinimal.csproj) and per-component interactivity application (NoInteractivityTests.cs that is running Components.TestServer.csproj that is not loading WasmMinimal assembly). The neatest solution was to put the common components is an external razor library. We could also start loading WasmMinimal in non-interactive tests but I decided it makes it too messy.
• I decided to break the rule of "no old tests should be removed" because old PageThatSetsNotFound and StreamingSetNotFound components were doing the same job as new tests but the new version is doing it in a
  more unified way, taking into consideration both POST and GET request, with and without enhanced navigation.

[ilonatommy]

it seems the tests cannot run rn, investigating

It might be connected to an issue with dotnet test, the previous attempt before dotnet mismatch finished with visible lack of test discovery:

[ilonatommy]

Adding no-merge until the tests won't be able to run fully. The PR can get an early review in the meantime.

[ilonatommy]

Failures connected with Router's streaming inferencing with enhanced navigation, e.g.:

• RedirectEnhancedGetToInternalWithErrorBoundary - turning on streaming SSR (via [StreamRendering] on the ) swaps out Blazor’s normal “diff-and-patch” path for a streaming pipeline that replaces the rendered subtree each time new HTML arrives. It happens only when NavigationManager.NavigateTo is used in ErrorBoundary in component's body (removal of boundary does not break enhanced nav). The test relies on keeping references to DOM and assuming enhanced navigation did not allow removing them. I am not sure why the rendering change happens only with ErrorBoundary.
• StopsProcessingStreamingOutputFromPreviousRequestAfterEnhancedNav, CanPerformStreamingRendering, CanNavigateToAnotherPageWhilePreservingCommonDOMElements - navigation with a NavLink in layout does not use enhanced navigation now but streaming.

Starting investigation on why the whole page, including constant elements, changes on streaming.

[ilonatommy]

In RedirectEnhancedGetToInternalWithErrorBoundary case the difference is how fast we reach quiesce. When Router is streaming, we are not getting into quite state so fast (what is preventing us? Don't know yet):

aspnetcore/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs

Line 153 in 1415d9b

if (!quiesceTask.IsCompletedSuccessfully)

and we trigger SendStreamingUpdatesAsync that, in contrast to EmitInitializersIfNecessary, starts the response. It's important in the context of how we handle the navigation. With Router that streams we fall into a _httpContext.Response.HasStarted=true path of processing navigation:
https://github.com/dotnet/aspnetcore/blob/1415d9bbe18c8ba97e9eb5a9618b5c149a261e3e/src/Components/Endpoints/src/Rendering/EndpointHtmlRenderer.EventDispatch.cs#L105C1-L116C10

Using HttpResponseStreamWriter explains why we loose references and change the way we render the page. Finding the root cause for prolonged quiesce wait could help eliminate this difference.

Edit:
forcing waitForQuiescence: true

aspnetcore/src/Components/Endpoints/src/RazorComponentEndpointInvoker.cs

Line 103 in 1415d9b

waitForQuiescence: result.IsPost || isErrorHandlerOrReExecuted);

fixes it but that's not a way to do it. It would break the scenarios where we really want to stream-in the changes.

[ilonatommy]

In case of the other category of failing tests: with NavLink click, it's a matter of

aspnetcore/src/Components/Components/src/RenderTree/RenderTreeDiffBuilder.cs

Line 21 in 1415d9b

public static RenderTreeDiff ComputeDiff(

method. We are entering it with oldTree as ampty array (why? Under investigation...) so logically, it builds the new view from scratch.
edit: this behavior is not influenced by Router streaming settings.

[ilonatommy]

Functional test failures are connected with empty streaming markers added to the response, e.g. Renders_RoutingComponent:
expected: "Router component\n<p>Routed successfully</p>"
actual:

Router component
<!--bl:3--><!--bl:4--><!--bl:5--><!--bl:6--><p>Routed successfully</p><!--/bl:6--><!--/bl:5--><!--/bl:4--><!--/bl:3-->

[ilonatommy]

The failing test have a difference in generated html, in component markers:
page 1st (with PageTitle):

page 2nd that we navigate to, (without PageTitle):

Because streaming state is inherited from parent component, PageTitle starts being in streaming state. Before adding StreamRendering to router was in non-streaming state and did not emit any marker in <!--/bl:{componentId}--> form. It can be fixed in tests, however it uncovers the consequences of using default, streaming router: all the components now will be streaming by default.

[ilonatommy]

Topics for to discuss in a follow-up:

• Do we want to treat POST same as "response started"? Are there any good workarounds? Router would be is able to render the page if we did not stop the renderer in this scenario.
• Don't we want to artificially add enhanced-nav=on headers to requests going through "not found processing"? We would have to respect user's setting of enhanced nav but if they did not disable it, we could prevent redirection in browser navigation or form submission cases.