Improve new stats error states: centering and unavailable sites

[adalpari]

Description

Follow-up from #23077.

Improves error handling and presentation across the new stats screens. Three related changes:

1. Center error messages in detail screens — the error Text in the Authors, Emails, Subscribers list, UTM, Year in review, and Most viewed detail screens (and the stats card) had no textAlign, so a message that wraps to multiple lines left-aligned instead of centering. Added textAlign = TextAlign.Center, matching the existing NoConnectionContent/TagsAndCategories convention.

2. Surface a clear message when stats are unavailable for the site — the WordPress.com stats API returns an invalid_blog error (e.g. file download stats on a Jetpack site) as a WpRequestResult.UnknownError with a 404. This was unhandled and fell through to the generic "An unexpected error occurred", discarding the API's reason. Added a lightweight, dependency-free parser for the API error code, a new StatsErrorType.NOT_AVAILABLE mapped to a meaningful string ("These stats aren't available for this site"), and classification of the invalid_blog code in both the UnknownError and WpError branches.

3. Hide the Retry button for unavailable stats — retrying a NOT_AVAILABLE error never succeeds, so an isNotAvailable flag is threaded through the card error chain (repository results, card ViewModels, StatsCardFetchResult, and the card/detail UI states). When set, only the message is shown — no Retry or WP Admin button — on both the stats card and the detail screen.

Unit tests added for the error-code parser and the NOT_AVAILABLE mapping.

Testing instructions

Error message centering:

1. Open Stats on any site and open a detail screen (e.g. Authors) while offline so the error state shows.

• Verify the (wrapping) error message is horizontally centered, not left-aligned.

Unavailable stats message + no Retry (requires a Jetpack/self-hosted site):

1. Log in to a Jetpack site where file download stats are unavailable.
2. Open Stats → Traffic and scroll to the File downloads card.

• Verify it shows "These stats aren't available for this site" (centered).

• Verify there is no Retry button on the card.

• Verify other stats work as expected

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

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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	pr23081-6a34720
	Build Number	1498
	Application ID	org.wordpress.android.prealpha
	Commit	6a34720
	Installation URL	7cas2f2jnq4e0

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

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

	App Name	Jetpack Android
	Build Type	Debug
	Version	pr23081-6a34720
	Build Number	1498
	Application ID	com.jetpack.android.prealpha
	Commit	6a34720
	Installation URL	4tr0sbatq4g98

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[codecov]

Codecov Report

❌ Patch coverage is 50.76923% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.37%. Comparing base (27c9a69) to head (6a34720).
⚠️ Report is 1 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...roid/ui/newstats/datasource/StatsDataSourceImpl.kt	0.00%	14 Missing ⚠️
...s/android/ui/newstats/mostviewed/MostViewedCard.kt	0.00%	5 Missing ⚠️
...droid/ui/newstats/authors/AuthorsDetailActivity.kt	0.00%	2 Missing ⚠️
...ui/newstats/mostviewed/MostViewedDetailActivity.kt	0.00%	2 Missing ⚠️
...ewstats/subscribers/emails/EmailsDetailActivity.kt	0.00%	2 Missing ⚠️
...s/subscriberslist/SubscribersListDetailActivity.kt	0.00%	2 Missing ⚠️
...press/android/ui/newstats/utm/UtmDetailActivity.kt	0.00%	2 Missing ⚠️
...ewstats/yearinreview/YearInReviewDetailActivity.kt	0.00%	2 Missing ⚠️
...ss/android/ui/newstats/datasource/StatsApiError.kt	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #23081   +/-   ##
=======================================
  Coverage   37.37%   37.37%           
=======================================
  Files        2339     2340    +1     
  Lines      126952   126991   +39     
  Branches    17499    17507    +8     
=======================================
+ Hits        47452    47469   +17     
- Misses      75618    75639   +21     
- Partials     3882     3883    +1     

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[nbradbury]

Fixes are good!