CMM-2132: Send the app language slug when fetching blogging prompts

[nbradbury]

Description

A Turkish customer reported that daily blogging prompts were appearing in English even though the app language was set to Turkish (CMM-2132).

After looking into this it appeared the problem was that we were requesting prompts using the language code tr_TR instead of the correct code (tr). This PR resolves this, but it doesn't fix the problem - which suggests a problem with the backend.

This change is still a legitimate correctness fix (sends the properly-formatted slug, consistent with Reader/Stats) and is a prerequisite for prompt localization to ever work, but the ticket also needs a backend follow-up to confirm/add prompt translations.

Testing instructions

Prerequisites: a WordPress.com or Jetpack site with the Blogging Prompts card enabled.

Verify the request format:

1. Set the app language to a non-English locale (Settings → App Settings → language, API 33+).
2. Open the My Site dashboard so the Blogging Prompt card loads.

• With Volley verbose logging (adb shell setprop log.tag.Volley VERBOSE), confirm the request is .../wpcom/v3/sites/{id}/blogging-prompts/?...&_locale=<slug> with a clean slug (e.g. tr), and that there is no duplicate _locale parameter.

Regression check:

3. Confirm the Blogging Prompt card and the Prompts list still load prompts normally in English.

• Verify prompts still appear and the card behaves as before.
• Verify existing unit tests pass (fluxc BloggingPromptsRestClientTest/BloggingPromptsStoreTest, WordPress FetchBloggingPromptsListUseCaseTest/BloggingPromptCardViewModelSliceTest).

[dangermattic]

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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	pr23057-f17af21
	Build Number	1497
	Application ID	org.wordpress.android.prealpha
	Commit	f17af21
	Installation URL	7fru0fk597ppg

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[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	pr23057-f17af21
	Build Number	1497
	Application ID	com.jetpack.android.prealpha
	Commit	f17af21
	Installation URL	30geb5cfmf0qg

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.44%. Comparing base (7e0ac61) to head (f17af21).

Files with missing lines	Patch %	Lines
...loggingprompts/BloggingPromptCardViewModelSlice.kt	28.57%	5 Missing ⚠️
...luxc/network/rest/wpcom/WPComGsonRequestBuilder.kt	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #23057   +/-   ##
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  Coverage   37.44%   37.44%           
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  Files        2326     2326           
  Lines      125606   125619   +13     
  Branches    17249    17249           
=======================================
+ Hits        47035    47043    +8     
- Misses      74748    74753    +5     
  Partials     3823     3823           

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