* Localization: AI translation primitives

Reusable, unit-tested Ruby primitives for the AI translation tier of the
localization pipeline — the service behind the `human ?? AI ?? English` floor
whose AI stub was left open in wordpress-mobile#25688. Pure prompt-building and validation with
the Anthropic SDK call injected, so the logic is testable without the gem or the
network. Not wired into any lane yet.

- TranslationValidator: format-specifier safety gate — a translation must
  preserve the source's placeholders (count and type; positional reordering
  allowed), or it is rejected and falls back to English.
- Glossary: brand do-not-translate list plus per-locale terms and register.
- AITranslator: single-string, per-key plural form-set (one consistent stem
  across CLDR forms), and batched string translation, with structured-output
  (output_config) enforcement.
- AnthropicBatch: Message Batches submit/await/results/collect for bulk backfill.

50 unit tests, rubocop clean.

* Localization: run the AI translation tooling unit tests in CI

The pure-Ruby unit suites (TranslationValidator, Glossary, AnthropicBatch,
AITranslator) weren't executed by any pipeline step — the "Unit Tests" jobs are
the Xcode/XCTest suites, and rubocop (via Danger) only lints them. Add a
lightweight Buildkite step that runs each fastlane/lanes/*_test.rb with plain
ruby (stdlib minitest — no Xcode, no app build, no bundle).

Runs unconditionally rather than behind should-skip-job.sh --job-type validation,
which skips on tooling-only changes — i.e. exactly the PRs that touch these files.

* Localization: correct the for_plural docstring

The previous note advertised for_plural as a one-line swap to wire the live
translation tier. That path routes each plural form through single-string
translate, so it forfeits the cross-form consistency translate_plural exists to
provide — the lemma drift PLURAL_OUTPUT warns about. Relabel for_plural as the
per-cell fallback and point the live-tier wiring at translate_plural's form-set
seam.

* Localization: stop the AI translator stripping quotes that are part of a translated value (wordpress-mobile#25721)

* Localization: assert clean() preserves quotes that are part of a value

A translation whose value is itself wrapped in quotation marks must keep them;
only the model's cosmetic wrapping around a raw single-string reply should be
stripped. Cover both structured paths (translate_plural, translate_all).

* Localization: stop clean() stripping quotes that are part of a value

clean() removes the cosmetic quotes a model wraps around a raw single-string reply. The plural and batch paths ran it on values already decoded by JSON.parse, so a value whose own content is quoted (e.g. "Reader") lost its quotes too.

Run clean() only on the raw single-string reply in translate(); the JSON-decoded plural/batch values are whitespace-trimmed but never quote-stripped, since JSON.parse has already removed the structural quotes and anything left is content.

Also covers the async collect_batch path (shares validated_batch) and curly-quoted values for free. Satisfies the tests added in d923c25.

* Localization: cover curly quotes and the batch path in the quote-preservation tests

Two more regression guards for the clean()-on-decoded-value fix: a curly/smart-quoted value (“Reader”) through translate_all, since clean() strips “ ” as well as straight quotes; and a quoted value through the async collect_batch path, which shares validated_batch with translate_all.

Both fail against the pre-fix code, so a narrower fix — only un-stripping straight quotes, or only the sync path — cannot slip past.

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Co-authored-by: Jeremy Massel <1123407+jkmassel@users.noreply.github.com>

* Localization: rename validated_batch/validated_forms to select_valid_*

At call sites the validated_ prefix reads as an adjective — "the batch that's
already been validated" — when both methods are in fact where batch and
plural-set translations run the placeholder gate, returning only the passing
subset. select_valid_batch / select_valid_forms make the filtering action plain
where they're called. Pure rename of two private helpers; no behavior change.

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Co-authored-by: Oguz Kocer <oguzkocer@users.noreply.github.com>