Feature request: Support AWS GovCloud and China partitions

**Is your feature request related to a problem? Please describe.**
The assessment tooling currently targets the standard AWS partition (`aws`) only. Customers operating in AWS GovCloud (US) (`aws-us-gov` partition) and the AWS China Regions (`aws-cn` partition) cannot run the AI/ML security assessment against their accounts. These partitions have distinct ARN formats, endpoint domains, service availability, and IAM/account boundaries, so code that assumes the commercial partition fails or silently misbehaves there.

**Describe the solution youd like**
Add first-class support for the GovCloud and China partitions so the assessment can be deployed and run in those environments. This includes:

- Partition-aware ARN construction (use the partition from the executing context rather than hardcoding `arn:aws:...`).
- Partition-aware service endpoints and the correct DNS suffixes (e.g. `amazonaws.com.cn` for China).
- Graceful handling of services/checks that are not available in a given partition (skip with a clear "not available in this partition" status rather than erroring).
- Region resolution that respects partition boundaries (GovCloud and China have their own region sets).
- Documentation and deployment templates (`template.yaml`, `template-multi-account.yaml`) updated to deploy cleanly in each partition.

**Describe alternatives youve considered**
- Maintaining separate forks/branches per partition — high maintenance burden and prone to drift.
- Manual, per-customer patching of ARNs and endpoints — error-prone and not reusable.

**Additional context**
Relevant areas likely to need changes: region resolution, the per-service assessment functions (Bedrock, SageMaker, AgentCore), the Step Functions state machine, and the CloudFormation/SAM templates. Service availability differs by partition (for example, some AI/ML services are not yet present in GovCloud or China), so per-check partition gating will be needed.

Start by tracing region resolution and the ARN and endpoint assumptions across the Bedrock, SageMaker, and AgentCore assessment functions. Then inspect the Step Functions state machine and template.yaml and template-multi-account.yaml for deployment and partition boundaries. Done means GovCloud and China deployments work, unavailable checks report a clear partition status, and commercial AWS behavior remains intact.

Written by the indexing model from the issue text.