test: expand operation coverage

[Iineman2]

/claim #71

Summary

• Expand src/operation.test.ts with focused coverage for execution paths that were still mostly uncovered.
• Cover raw result transformation through registry hooks, internal empty-result handling, Hyperdrive success/error handling, External API SDK fallback, and every supported SDK connection branch.
• Keep this test-only and non-overlapping with the active CSV import, RLS, cache, and allowlist coverage PRs.

Validation

• .\node_modules\.bin\prettier.cmd --check src/operation.test.ts - passed
• .\node_modules\.bin\vitest.cmd run src/operation.test.ts --coverage.enabled true --coverage.include src/operation.ts --coverage.reporter text - passed; src/operation.ts reports 94.21% statements, 89.47% branches, 100% functions, 94.06% lines
• .\node_modules\.bin\vitest.cmd run - operation tests pass; current main still has the existing 4 failing src/rls/index.test.ts expectations that are unrelated to this operation-only change
• git diff --check - passed

Demo video

• Short proof video: https://github.com/Iineman2/starbasedb/releases/download/starbasedb-71-maintenance-demos/starbasedb-pr153-operation-coverage-demo.mp4
• This is backend/unit-test-only coverage, so the video is a reviewer walkthrough of the covered execution paths and local verification rather than a UI flow.

[Iineman2]

Added the requested short demo/proof video to the PR description: https://github.com/Iineman2/starbasedb/releases/download/starbasedb-71-maintenance-demos/starbasedb-pr153-operation-coverage-demo.mp4