Brev 9311/fix invalid instance type error

[arush070]

Problem

brev create --type <x> printed the backend's internal message
cloudCredId or workspaceGroupId must be specified whenever:

• the instance type was unrecognized (typo), or
• the type was valid but had no capacity for the caller's org.

Change

Two-layer validation, both in CLI code (no parsing of backend strings):

1. Pre-flight skip in createInstancesWithType
   • New validateInstanceTypeAvailability(spec.Type) returns breverrors.ValidationError with one of:
     • instance type "<x>" is not a recognized type; run 'brev search' to see available types
     • instance type "<x>" is currently unavailable (no capacity); try again later or run 'brev search' to find another type
   • Logged as Skipping: <message> so the user sees the type once.
   • Bypassed for --launchable (launchables supply their own workspace group and may reference types outside the org listing).
2. Structural pre-condition in createWorkspace
   • If cwOptions.WorkspaceGroupID == "" after all resolution paths (instance-type lookup, launchable config), return a ValidationError before the API call. Separate message when the listing API itself was unavailable, so the user can retry vs. fix
     the type.
3. Supporting helper AllInstanceTypesResponse.HasInstanceType in pkg/cmd/gpusearch/gpusearch.go so the validation can tell "unknown" from "known but no groups".

Tests

New cases:

• TestValidateInstanceTypeAvailability (5 subtests: nil listing, valid group, unknown type, known-but-no-groups, error is ValidationError)
• TestCreateInstancesWithTypeSkipsInvalidType
• TestCreateInstancesWithTypeSkipsUnavailableType
• TestCreateInstancesWithTypeBypassesValidationForLaunchable
• TestAllInstanceTypesResponseLookup (5 subtests forGetWorkspaceGroupID + HasInstanceType)