Fix push gateway with some push provider (Sunup/autopush)

[p1gp1g]

Content

This PR adds a few HTTP codes that imply the push server doesn't have any matrix gateway: it returns NoMatrixGateway instead of Error.

Fixes: #5723

Motivation and context

Autopush (Sunup push server) returns a 406 to the matrix gateway discovery request (GET to /_matrix/push/v1/notify), which is currently considered as a network error and therefore fallback to the stored push gateway if it exists or to the custom gateway *.

* It should fallback to the default gateway - but I'm opening another PR for this, in case there is a need I am not aware of

Tests

• Install Sunup
• Enable UnifiedPush and selection Sunup
• Go to notification troubleshooting list
• Observe the nonexistent gateway being used

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

[github-actions]

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[jmartinesp]

Thanks, this fixed my issues with using Sunup! @bmarty since you're the most knowledgeable around unified push, would you like to take a look too?

[jmartinesp]

Super nitpick: since this isn't a constant, it should start with lower case. We can change this later though.

[jmartinesp]

It seems like there are some unit tests to fix: https://github.com/element-hq/element-x-android/actions/runs/19391981774/job/55563077771?pr=5741

[p1gp1g]

Oh yeah, I forgot to patch it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.71%. Comparing base (e22c976) to head (241fcb0).
⚠️ Report is 183 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5741      +/-   ##
===========================================
- Coverage    79.71%   79.71%   -0.01%     
===========================================
  Files         2422     2422              
  Lines        65059    65066       +7     
  Branches      8303     8304       +1     
===========================================
+ Hits         51861    51865       +4     
- Misses       10225    10229       +4     
+ Partials      2973     2972       -1     

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[bmarty]

LGTM, thanks!