feat(BREV-9100): add --region flag to brev create and brev search

[hakhandelwal11]

• Adds -r, --region flag to brev create for targeting a specific region/location when launching an instance. The value flows through to cwOptions.Location on the create request.
• Adds -r, --region flag to brev search (gpu and cpu) for filtering catalog results by region. Matching is case-insensitive substring against AvailableLocations.
• Exposes available_regions in the brev search --json payload so users can discover valid regions.

[theFong]

what QA have we done?

[hakhandelwal11]

what QA have we done?

@theFong ,
We did a local build of the CLI and ran through the flows to validate behavior

• ./brev search gpu --region us-east-1: confirmed exact and partial (substring, case-insensitive) matches against AvailableLocations.
• ./brev gpu create --region <valid>: instance launches, verified Region lands on the workspace.
• ./brev gpu create --region <bogus>: hits validateRegionExists and errors out before any API mutation.
• ./brev gpu create --instance-type <t> --region <r-not-supported-by-t>: hits validateRegion with the "not available for instance type(s)…" error.
• ./brev gpu create --instance-type <unknown> --region <r>: hits the unknown-type branch.