fix(sfcompute): stop sending deprecated InfiniBand field

[masonJamesWheeler]

Context

SF Compute now derives InfiniBand passthrough from the authenticated account's effective SKU configuration. The legacy request field is deprecated and ignored, and keeping this client-side writer would leave Brev sending obsolete policy input on every instance create.

Changes

• Remove the legacy _preview_enable_infiniband field from the SF Compute request body.
• Assert that the Brev contract no longer emits the field.

This is rollout-safe with current and new SF Compute servers: omission already defaults to false, and the compatibility API continues accepting older clients that still send the field.

Testing

• make vet fmt-check test
• go test ./v1/providers/sfcomputev2