[bug] aws rhel-ai create: AMI architecture hard-coded to x86_64, ARM64 instance types fail

When using aws rhel-ai create with ARM64 instance types (e.g. Graviton-based g5g.*), the provisioning fails because the AMI lookup is hard-coded to x86_64. The selected AMI architecture doesn't match the requested instance architecture.

Steps to Reproduce

mapt aws rhel-ai create \
  --project-name test \
  --backed-url file:///workspace \
  --conn-details-output /workspace \
  --version 3.4.0 \
  --spot --spot-eviction-tolerance medium \
  --compute-sizes g5g.xlarge,g5g.2xlarge,g5g.4xlarge

Error

error: sdk-v2/provider2.go:571: sdk.helper_schema: creating Auto Scaling Group (main-araid-asg-19f8de4):
operation error Auto Scaling: CreateAutoScalingGroup, api error ValidationError:
You must use a valid fully-formed launch template. The architecture 'arm64' of the specified instance type
does not match the architecture 'x86_64' of the specified AMI.

Root Cause (Opus says...)

The AMI architecture is hard-coded to "x86_64" in pkg/provider/aws/action/rhel-ai/constants.go:25 and used in all AMI lookups. Unlike aws rhel create, the aws rhel-ai create command does not expose an --arch flag.

Specifically:

• constants.go:25 defines amiArch = "x86_64" as a package-level variable
• rhelai.go lines 136, 140, 288-298 all use this hard-coded value for AMI filtering
• cmd/mapt/cmd/aws/hosts/rhelai.go does not register a --arch flag (compare with rhel.go:77)
• The RHELAIArgs.Arch field exists but is never populated from CLI flags, and its value is stored in rhelAIRequest.arch but never used

Expected Behavior

aws rhel-ai create should select the correct AMI based on the VM type it selects, or support an --arch flag, and use it when filtering AMIs/selecting instance types.

Start with pkg/provider/aws/action/rhel-ai/constants.go:25 and the AMI lookup and instance-selection logic in rhelai.go:136, 140, and 288-298. Compare CLI architecture handling in cmd/mapt/cmd/aws/hosts/rhelai.go with rhel.go:77, then trace RHELAIArgs.Arch and rhelAIRequest.arch. Done means ARM64 instance types select a matching AMI, or an --arch value is consistently supported through filtering and instance selection.