[cinder-csi-plugin] Validate volume status before attach

[hemna]

What this PR does / why we need it:

AttachVolume now validates that the volume is in an acceptable state before calling the Nova volume attach API.

• For regular (single-attach) volumes: status must be available
• For multi-attach capable volumes: status must be available or in-use
• migration_status must not be migrating

Previously, the driver would call volumeattach.Create regardless of volume status, relying on Nova/Cinder to reject the request. This produced opaque 409 Conflict errors that made troubleshooting difficult and generated unnecessary API calls for volumes in transitional states (e.g., creating, downloading, detaching).

Additionally, a volume can have status available while actively being migrated (migration_status: migrating). Attempting to attach in this state would fail with an opaque Cinder error. The driver now checks this explicitly.

With this change the driver returns immediate, descriptive errors when the volume is not attachable, allowing the CO (external-attacher) to retry with exponential backoff via standard CSI error handling.

How it works:

A custom volumeForAttach struct is used to extract both the standard volume fields and migration_status from the Cinder API response in a single API call. This is necessary because gophercloud's Volume struct does not map the migration_status field.

The validation is placed inside AttachVolume (after the existing idempotency check for already-attached volumes). This means:

• No additional API calls are introduced (replaces the existing GetVolume call)
• No new interface methods are required
• No changes to mock/fakecloud implementations are needed

Which issue this PR fixes (if applicable):
Related to #3124

Special notes for reviewers:

This is a simpler alternative to the polling/waiting approach in #3124. Rather than adding a wait-loop with poll intervals and context deadline handling, this PR fails fast with a clear error and lets the CO handle retries — which it already does via exponential backoff.

The DetachVolume function already performs similar status validation (lines 307-318 of openstack_volumes.go), so this brings AttachVolume to parity.

The validation order is:

1. Already attached to same instance? → return early (idempotent)
2. migration_status == "migrating"? → reject
3. Volume status acceptable for attach type (single vs multi-attach)? → reject if not

Release note:

[cinder-csi-plugin] AttachVolume now validates volume status and migration_status before calling the Nova attach API, returning clear errors for volumes not in an attachable state.

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