fix(provider): return valid instance statuses

[rm-you]

Nova servers in BUILD or DELETING are currently reported as pending_create or pending_delete, which GARM rejects in provider results. A single such server can fail validation of the entire ListInstances response and prevent pool or scale-set reconciliation. Other unmapped Nova states currently produce an empty status, which GARM also rejects.

Use typed constants for the existing ACTIVE, SHUTOFF, and ERROR mappings, and return InstanceStatusUnknown for all other states. Add table-driven conversion coverage and extend the existing HTTP list test with building and deleting servers alongside a running server.

Validation: the regression tests fail before the fix and pass afterward; the full vendored test suite passes with the race detector, as do repository lint, formatting, and dependency consistency checks.