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.

Start with docs/design/google-sheets/testing.md and review dependencies #399–#404 to understand the seams and test-project setup. Use the acceptance criteria as the completion checklist: the unit, fake Google, ClickHouse, browser, scale, real Google, and CI-secret cases should be covered, with clear skipping when the external project is unavailable.