Fix NotFoundSetOnFormSubmit_ResponseNotStarted_SSR test and enhanced navigation timing issue

[Copilot]

This PR addresses a timing issue in enhanced navigation where NotFound calls made synchronously could reach the enhanced navigation code before proper initialization, causing undefined access errors.

Changes Made

1. Test URL Correction

Fixed NotFoundSetOnFormSubmit_ResponseNotStarted_SSR test to use the correct URL:

• Before: /post-not-found-ssr-streaming (streaming enabled)
• After: /post-not-found-ssr (streaming disabled)

The test name explicitly mentions "ResponseNotStarted" which should test non-streaming scenarios where the response hasn't started streaming yet.

2. Enhanced Navigation Timing Fix

The core issue was a race condition where synchronous NotFound calls could trigger performEnhancedPageLoad before navigationEnhancementCallbacks was initialized.

Root Cause: When forms submit and immediately call NotFound, the enhanced navigation code path executes before the boot function completes initialization.

Solution:

• Moved enhanced navigation initialization earlier in the boot function
• Changed navigationEnhancementCallbacks from direct assignment to a promise-based pattern
• performEnhancedPageLoad now awaits the callbacks promise, ensuring they're available before use

Fixes #62540.

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[dotnet-policy-service]

Greetings human! You've submitted a PR that modifies code that is shared with https://github.com/dotnet/runtime . Please make sure you synchronize this code with the changes in that repo!

[ilonatommy]

The underlying issue should be fixed by #62524. This PR will only rename the test and double check if the missing test case is fine after the fix lands.