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)

[arush070]

Hi @patelspratik ,

Thanks for your previous approval on this PR.
As this PR remained open while main continued to move ahead, some merge conflicts came up. We have now resolved those conflicts and rebased the PR onto the latest main, while keeping the same changes that were previously approved.
Could you please review it once more? Once approved, please let us know if we are good to merge.

Thanks in advance.