mpg: route database list/create through Machines API

[tripledoublev]

Summary

• route MPGv2 database list / database create through the Machines API
• fall back to the legacy MPG API only on 404 (cluster not served by the Machines API); other errors now surface
• bump fly-go to v0.9.9 and add coverage for the 404 fallback, error paths, and JSON/table output compatibility

Uses the database endpoints added in superfly/fly-go#288.

Testing

• go test ./... -count=1, race tests on affected packages, vet, lint, go generate check, build
• live validation against a disposable MPG cluster: list (table + JSON), create + verify, destroy

[tomr-a5]

Looks good to me!