ContributionMilestoneTests fails locally on non-US locales (31.000 vs 31,000)

[esteugene]

What happens

ContributionMilestoneTests → "share message and URLs include encoded milestone content" fails on
any machine whose locale groups thousands with . rather than ,:

Expectation failed: (message → "I've dictated 31.000 words with Muesli. …") == "I've dictated 31,000 words with Muesli. …"
Expectation failed: (tweetURL.absoluteString → "…31.000%20words…").contains("31,000")

The test asserts a US-grouped 31,000, but the value is formatted with the current locale, so on a
German, Swiss, Polish, Russian, Dutch (and so on) system it renders 31.000 and the two assertions
fail. CI runs on a US locale, so the suite is green there and only contributors see it.

Reproduce

defaults write -g AppleLocale -string "de_CH"   # or run on a machine already set to such a locale
swift test --package-path native/MuesliNative --filter ContributionMilestone

Observed on main at c0a73d7, macOS 27, locale pl_PL with de_CH regional formats:
Test run with 11 tests in 1 suite failed with 2 issues.

Suggested fix

Either format the expectation with the same locale-aware formatter the production code uses, or pin
the formatter in the test to a fixed locale (Locale(identifier: "en_US_POSIX")) so the assertion
does not depend on the developer's region.

I noticed this while running the suite for #433 / #434 and confirmed it fails identically on
unmodified main, so it is not caused by those branches. Happy to send a patch if you have a
preference between the two approaches.

[pHequals7]

Closing as a duplicate of #343 (same root cause: ContributionSocialShare.formatCount uses the current locale for fixed English copy). The fix is in flight in #345, which has just been rebased and validated. Thanks for the crisp report — the 31.000 vs 31,000 example matched #343 exactly.