fix(discover): Prevent 404 for disabled homepage feature

[sentry]

This PR addresses the "Failed HTTP Operation" issue (JAVASCRIPT-384Z) occurring on /explore/discover/homepage/.

Root Cause:
The backend endpoint GET /api/0/organizations/{org}/discover/homepage/ was returning an HTTP 404 Not Found status when the requesting organization did not have either the organizations:discover or organizations:discover-query feature flags enabled. The frontend, however, was only checking for organizations:discover-query before making the API call. This mismatch, combined with the backend's 404 response for a feature-gated endpoint, led to Sentry's performance monitoring flagging it as a "Failed HTTP Operation."

Changes Made:

1. Backend (src/sentry/discover/endpoints/discover_homepage_query.py): Modified the get, put, and delete methods of DiscoverHomepageQueryEndpoint to return HTTP_204_NO_CONTENT instead of HTTP_404_NOT_FOUND when the required feature flags (organizations:discover or organizations:discover-query) are not present. This aligns the behavior with the case where no saved homepage query exists, which is a non-error state the frontend can handle gracefully.
2. Frontend (static/app/views/discover/homepage.tsx): Updated the enabled condition for the useApiQuery hook to check for both organization.features.includes('discover-query') OR organization.features.includes('discover'). This ensures the frontend only attempts to fetch the homepage query if the organization has any of the necessary Discover feature flags enabled, preventing unnecessary API calls that would result in a 204 response.

Impact:
These changes eliminate the erroneous 404 responses, preventing the

Legal Boilerplate

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Fixes JAVASCRIPT-384Z

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀