feat: add region-aware instance search and creation

[theFong]

Summary

• add --region/-r to GPU and CPU search with case-insensitive partial matching for discovery
• add exact, case-insensitive placement selection to brev create through --region or --location, plus optional --sub-location
• include CPU types in create-time validation so types such as n2d-standard-2 can be placed in us-west2
• validate requested locations before provisioning and list the supported locations on mismatch
• expose default location and sub-location in search tables and JSON
• show selected or default placement in create dry-run and pre-create output
• resolve authenticated cloud credentials by instance type and location
• safely override or clear launchable sub-locations when the CLI location changes

Issue

Fixes #454

This covers the issue end to end: the request fields are populated, CPU locations are validated, default placement is visible before provisioning, and the reported n2d-standard-2 / us-west2 / us-west2-a path has a mocked request-level regression test.

Compatibility

• brev search and brev create without placement flags retain their existing selection behavior
• piped tables keep TYPE and TARGET_DISK as the first two columns; PROVIDER also remains third
• new JSON fields are additive and omitted when unavailable
• search remains forgiving for discovery, while create requires an exact catalog location
• GetCloudCredID preserves first-match behavior when no location is requested
• fallback chains remain usable when at least one candidate supports the requested location
• private launchable types absent from the authenticated catalog retain their configured credential

Dedupe investigation

The issue suggestion to revisit uniqueInstanceType=true was reviewed. This PR leaves it unchanged: validation uses the public catalog available_locations, and authenticated selection already matches both type and location across the returned credential entries. Changing server response cardinality without a reproduced failure would expand the scope and risk of this fix.

Validation

• go test -count=1 ./pkg/cmd/gpusearch ./pkg/cmd/gpucreate
• go test -race ./pkg/cmd/gpusearch ./pkg/cmd/gpucreate
• go vet ./pkg/cmd/gpusearch ./pkg/cmd/gpucreate
• golangci-lint run ./pkg/cmd/gpusearch ./pkg/cmd/gpucreate
• go build ./...
• go test -count=1 ./pkg/integration

No real instance was provisioned while validating this change.

Prior work

This is a clean refresh of #393 against current main, building on the original region-flag work by @hakhandelwal11. The older PR is intentionally left open for maintainers to handle.