test: cover durable object bootstrap helpers

[TechEnthusGH]

Summary

• Add focused Vitest coverage for StarbaseDBDurableObject bootstrap behavior.
• Cover constructor-created internal temp tables (tmp_cache, allowlist tables, RLS policies).
• Cover init() returning bound helper methods and raw query cursor metadata for parameterized queries.

This is intentionally test-only and scoped to src/do.test.ts.

Bounty

/claim #71

Validation

• corepack pnpm exec vitest run src/do.test.ts -> 9 tests passed
• corepack pnpm exec prettier --check src/do.test.ts -> passed
• git diff --check -> passed
• corepack pnpm exec vitest run --coverage --coverage.reporter=json-summary --coverage.reporter=text-summary -> existing unrelated RLS baseline failures remain; this PR improves src/do.ts coverage from 41.17% to 51.96% lines, 37.5% to 62.5% functions, and 17.64% to 29.41% branches.

Full-suite baseline note: current main already fails four src/rls/index.test.ts assertions and the global branch threshold before this PR's changes.

Demo

This is a test-only coverage PR; the focused terminal verification above demonstrates the change.