Replace site-settings snackbar with My Site connectivity banner

[jkmassel]

Description

Follow-up to #22785 (review feedback from @nbradbury). The snackbar from a failed editor capabilities fetch is non-actionable and disappears, leaving the user with no way to recover. On a cold launch with flaky network, it pops up before the user has done anything.

This PR moves the fetch into a new SiteConnectivityBannerViewModelSlice that posts a persistent SingleActionCard banner into the My Site header (last position, below the reauth banner) when the fetch fails AND there's no cache. Tapping the banner retries the fetch (bypassing the per-session dedup). Successful fetch — or a cached value — clears the banner.

The banner copy is generic — "Unable to connect to your site. Some functionality might be limited." — because the apiRoot fetch failure signals broader site reachability problems, not just editor issues.

Why a slice?

MySiteViewModel already carries LargeClass/LongMethod/LongParameterList suppressions. The fetch is logically self-contained (takes a SiteModel, posts a card or null), so it makes sense to extract. The slice owns the per-site session dedup that #22833 introduced inline — same behavior, dedicated home, dedicated tests.

Testing instructions

Covered by 9 new unit tests in SiteConnectivityBannerViewModelSliceTest:

• Fetch succeeds → banner null
• Fetch fails with no cache → banner shown
• Fetch fails with cached value → banner null (silent)
• Background dedup after success
• Background retry after failure (no dedup until success)
• User-initiated bypasses dedup
• Banner retry tap bypasses dedup and clears banner on success
• clearBanner() clears
• Two different sites → both fetch (no cross-site dedup)

Manual:

1. Put the device in airplane mode and cold-launch the app (without any saved data – you'll probably have to delete and re-install it), then go to My Site.

• Verify the connectivity banner appears at the top of My Site (below the site header, below the reauth banner if present).
• Verify there is no snackbar.

2. Re-enable connectivity and tap the banner.

• Verify the fetch runs and the banner disappears on success.

3. Background the app and reopen with a working network.

• Verify the banner does not appear.

4. With a site that has cached capabilities, simulate a network failure on the next fetch.

• Verify the banner does NOT appear (silent failure when we have data).

Related

• Follow-up to Add site settings for third-party blocks and theme style gating #22785
• Builds on Skip repeat editor capability fetches in MySiteViewModel #22833 (now merged)

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 26.8. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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	App Name	Jetpack Android
	Build Type	Debug
	Version	pr22834-ac1833c
	Build Number	1488
	Application ID	com.jetpack.android.prealpha
	Commit	ac1833c
	Installation URL	5k5ibp6iugcb8

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Android by scanning the QR code below to install the corresponding build.

	App Name	WordPress Android
	Build Type	Debug
	Version	pr22834-ac1833c
	Build Number	1488
	Application ID	org.wordpress.android.prealpha
	Commit	ac1833c
	Installation URL	0gfphu7e70vqg

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🤖 Build Failure Analysis

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[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.32%. Comparing base (0a2a007) to head (ac1833c).

Files with missing lines	Patch %	Lines
...nnectivity/SiteConnectivityBannerViewModelSlice.kt	89.28%	0 Missing and 3 partials ⚠️
...org/wordpress/android/ui/mysite/MySiteViewModel.kt	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #22834   +/-   ##
=======================================
  Coverage   37.31%   37.32%           
=======================================
  Files        2319     2320    +1     
  Lines      124558   124571   +13     
  Branches    16920    16924    +4     
=======================================
+ Hits        46484    46496   +12     
+ Misses      74319    74315    -4     
- Partials     3755     3760    +5     

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[nbradbury]

@jkmassel This is an improvement, but if I enable airplane mode immediately after logging in, I see three separate warnings about connectivity.

Is there any way we can reduce the number of warnings? Perhaps skip showing the slice when the problem is lack of connectivity, since we know the "No connection" banner will appear.