Test InputLanguage without modifying the Windows language list

[Eales]

Fixes #13274

cc @LeafShi1: this issue is still assigned to you. I did not find an open PR touching
these files; please let me know if there is ongoing work to coordinate with. Opening
as a draft for that coordination and review of the small internal testability extraction.

Proposed changes

• Replace the supplemental-language tests' PowerShell install/uninstall steps with
  deterministic tests of the existing language-tag mapping. Tests no longer call
  Set-WinUserLanguageList or modify the user's installed language list.
• Extract an internal mapping helper with a per-call language source. Cover all four
  transient keyboard IDs with got-Goth, jv-Java, zgh-Tfng, and nqo, plus
  non-transient bypass, first-match selection, and the existing CultureInfo fallback
  for empty/nonmatching sources. Production registry lookup remains lazy and ordered.
• Use installed layouts for FromCulture, current-language, and associated event-args
  tests. Handle multiple layouts sharing a culture without assuming that FromCulture
  selects a particular handle other than the first matching installed layout.
• Restore the original current input language in finally, on success and failure.
  The old setter test's catch swallowed assertion failures. Remove event-args tests'
  early returns that could pass without exercising the constructor.

Customer Impact

• Contributors can run these tests without adding/removing Windows languages or
  having an en-US input layout installed.
• Supplemental mapping coverage no longer depends on Windows applying a language-list
  update before the next assertion. Real installed-layout integration coverage remains.

Regression?

• No new product behavior is intended; this addresses existing test issues.

Risk

• Low: four InputLanguage-related files only; no public API changes or global test hook.
• The internal extraction preserves the transient-ID gate, registry order, first-match
  behavior, fallback, and disposal. It does not change FromCulture or layout activation.

Test methodology

• Based on main at 8efd9a920dba90722f8db1f7e943f74837d26579.
• Baseline InputLanguage* selection: 34 passed, 4 failed. The four event-args cases
  required an unavailable en-US input language. The old supplemental installer test
  was deliberately not executed because it changes the real Windows language list;
  this is not a claim to have reproduced the installation timing race in Random failures in InputLanguage_FromCulture_SupplementalInputLanguages_Expected #13274.
• Patched InputLanguage* selection: 64 passed, 0 failed, 0 skipped with each of
  --culture pl-PL and --culture en-US, without an exclusion filter.
• Controlled mutation checks: removing the fourth transient ID caused six mapping
  failures; an injected setter assertion was reported with finally but incorrectly
  passed with the old catch policy. All temporary mutations were removed.
• Release build: zero warnings and errors. No retries or skip conditions added.
  Validation is targeted; the full repository test suite was not run for this PR.

Targeted invocation after building the unit-test project:

.\.dotnet\dotnet.exe artifacts\bin\System.Windows.Forms.Tests\Release\net11.0-windows7.0\System.Windows.Forms.Tests.dll `
  --culture pl-PL --filter-class 'System.Windows.Forms.Tests.InputLanguage*'

Test environment(s)

• Windows 11 25H2 x64, build 26200.9168; Polish input layout, no en-US input layout.
• SDK 11.0.100-rc.1.26420.103; test runtime 12.0.0-alpha.1.26458.117 as required by main.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.24166%. Comparing base (8efd9a9) to head (3a2bf24).

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15076   +/-   ##
=============================================
  Coverage   37.24166%   37.24166%           
=============================================
  Files            246         246           
  Lines           9774        9774           
  Branches        1029        1029           
=============================================
  Hits            3640        3640           
  Misses          5970        5970           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.24166% <ø> (ø)
production	39.36526% <ø> (ø)
test	20.64923% <ø> (ø)
unit	39.36526% <ø> (ø)

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