GS: End-to-end Google Sheets integration test suite

Parent: #398
Depends on: #399, #400, #401, #402, #403, #404

Design:

• docs/design/google-sheets/testing.md

Scope

Add the complete automated test matrix for Google Sheets integration using injected seams, a dedicated Google Cloud test project, disposable spreadsheets, supported browsers, and scale fixtures.

Acceptance criteria

• Unit tests cover binding/checkpoint codecs, validation, conversion, limits, version checks, capability probes, and cursor query construction.
• Fake Google tests cover exact-range writes, metadata, staging, publication, retries, ambiguous responses, copies, renames, deletion, permissions, and cleanup.
• ClickHouse seam tests cover binding storage, table conformance, typed parameters, UUID/schema/query validation, upper-bound capture, and append pages.
• Chromium, Firefox, and WebKit cover deep links, OAuth popups, auth redirects, cancellation, tab close, concurrency, full refresh, append resume, repair, and unlink.
• Scale tests prove bounded memory and enforce row/cell/byte/cell-length limits without truncation.
• Real Google tests use only disposable project-owned files and leave diagnostic metadata when cleanup fails.
• CI secrets are isolated and tests skip clearly when the external test project is unavailable.

Non-goals

Production monitoring or load testing against personal Google accounts.

docs/design/google-sheets/testing.md から始め、依存関係 #399–#404 を確認して、接続部分とテストプロジェクトのセットアップを理解します。受け入れ基準を完了チェックリストとして使用してください。unit、fake Google、ClickHouse、browser、scale、real Google、CI-secret のケースをカバーし、外部プロジェクトが利用できない場合は明確にスキップする必要があります。