fix(adapters): add ja translations to built-in Google/Meta account credential field labels (#237)

[hyoshi]

Summary

In the Japanese configure UI, the built-in Google Ads / Meta Ads per-account field labels stayed English ("Customer ID", "Login Customer ID (MCC)", "Ad Account ID") while plugin-declared fields localize. The built-in AccountCredentialField declarations simply never used the #186 i18n hooks.

Changes

Adds display_name_i18n / description_i18n {"ja": ...} maps to the three built-in declarations, exactly per the issue's table:

key	ja label
customer_id	カスタマー ID
login_customer_id	ログインカスタマー ID（MCC）
account_id	広告アカウント ID

Japanese descriptions are faithful translations of the English hints (review-verified: the login_customer_id semantics — MCC header use, blank-inherits-default, parent_id auto-population — carry over without drift). No consumer changes: the locale-resolved discovery path (i18n[locale] → i18n["en"] → display_name) already picks these up, and en keeps resolving to the bare display_name.

Test plan

• tests/adapters/google_ads/test_adapter.py / tests/adapters/meta_ads/test_adapter.py — new pins for the exact ja labels, unchanged en fallback, non-empty ja descriptions (non-empty matters: the resolver treats "" as undeclared)
• 229 tests pass across tests/adapters + plugin-credentials suites; ruff + black clean
• CI green on the PR

Closes #237