test: cover SDK database adapters

[davidc-alt]

Summary

• add focused Vitest coverage for executeSDKQuery adapter selection
• cover PostgreSQL, MySQL, Cloudflare D1, Starbase, and Turso construction and query forwarding
• cover missing external configuration and unsupported provider failure paths
• keep the change test-only and separate from the existing operation hook coverage PR

Coverage impact

Against current main, the full coverage run changes:

• statements: 76.82% → 78.59%
• branches: 47.68% → 51.08%
• functions: 82.10% → 83.53%
• lines: 77.17% → 79.01%
• src/operation.ts branches: 28.94% → 57.89%

Verification

• ./node_modules/.bin/vitest run test/operation.sdk.test.ts — 7/7 passed
• ./node_modules/.bin/vitest run --exclude src/rls/index.test.ts — 151/151 passed across 19 files
• ./node_modules/.bin/prettier --check test/operation.sdk.test.ts — passed
• git diff --check — passed

The complete suite still has the four pre-existing src/rls/index.test.ts failures visible on current main, and the repository-wide branch threshold remains below 75%. This patch introduces no additional test failures.

Demo

Short SDK adapter test walkthrough

/claim #71

[davidc-alt]

Withdrawing this bounty claim after confirming issue #71 is already labeled 💰 Rewarded and Algora lists no open Outerbase bounties. The focused tests are valid, but I do not want to create unpaid review work under a completed bounty.