feat(connections): add desktop DatabaseType constants to TableProModels (Phase 1 prep)

[datlechin]

Summary

Phase 1 (R-002) step 8.1: bring the authoritative TableProModels.DatabaseType to constant parity with the desktop app's duplicate, so the desktop type can be removed in a later PR without breaking call sites. Package-only and additive.

• Adds the 3 constants the desktop type has but the package lacked: .cockroachdb ("CockroachDB"), .scylladb ("ScyllaDB"), .turso ("Turso").
• Leaves the built-in allKnownTypes list at 17 (mobile's type picker is unchanged); the new constants are intentionally outside it.
• Does not touch allKnownTypes (static), pluginTypeId/iconName (hardcoded). Mobile and the TableProCore library modules depend on those as-is.

Risk Addressed

• R-002: duplicate DatabaseType definitions (groundwork; full consolidation is the follow-up).

Seam note for the follow-up (8.2)

The desktop type's allKnownTypes, pluginTypeId, and iconName are registry-driven; the package's are static/hardcoded. Swift extensions can't override static/instance members, so when the desktop adopts this type it must install the registry-driven behavior via a resolver hook (or distinct names), not as overrides. The desktop .bigQuery vs package .bigquery constant-name mismatch is reconciled there too.

Verification

• swift test --package-path Packages/TableProCore (61 tests pass; new DatabaseType cases included)
• scripts/audit-refactor-health.sh --check (contract-drift gate clean)
• Codable form unchanged (string rawValue), so persisted connection JSON is unaffected.

Notes

• ABI bump required: no.
• Docs updated: no.
• macOS app untouched (it does not import TableProModels yet).