Fix(Paywalls): Avoid POSIX localizations (#4048)

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### Checklist
- [x] If applicable, unit tests
- [ ] If applicable, create follow-up issues for `purchases-ios` and
hybrids

### Motivation
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<!-- Please link to issues following this format: Resolves #999999 -->

Certain android devices seem to present a POSIX locale and it results in
unexpected string output to the user.

For example, the expectation would be "$1.99" but the user sees "$ 1.99"
[linear issue
PW-1345](https://linear.app/revenuecat/issue/PW-1345/normalize-android-generated-price-locale-selection)
### Description
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Filters out posix if possible


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Localized paywall currency formatting only; behavior is narrowed to
avoid POSIX locales with no auth or payment impact.
> 
> **Overview**
> Fixes incorrect paywall price strings on some Android devices (e.g.
**$ 1.99** instead of **$1.99**) by changing how the paywall picks a
**currency locale** for the storefront country.
> 
> Storefront-country locale lookup is moved into
**`getAvailableStorefrontCountryLocalesByLanguage`**, which **drops
POSIX variants** when building the per-language map so currency
formatting matches store-style spacing. Non-POSIX regional variants
(e.g. Valencian) keep the same selection behavior.
> 
> Unit tests cover POSIX exclusion and non-POSIX variant preservation.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
90d109a54c62910a3cf5ff02b3af23b1854ec718. Bugbot is set up for automated
code reviews on this repo. Configure
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<!-- /CURSOR_SUMMARY -->

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Co-authored-by: Álvaro Brey <alvaro.brey@revenuecat.com>