Bad error when region has a space in it

## Bug Report
When `AWS_REGION` or `AWS_DEFAULT_REGION` has a space in it, the SDK will refuse to construct a request in a very opaque way:
```
operation="ListTables" service="dynamodb" status="construction_failure" EndpointResolutionError(InvalidUri(InvalidUriChar))
```

Since this is actually a pretty common way to have accidentally broken environment variables, we should error out on this specific case.

To reproduce:
`export AWS_REGION=' us-east-1' and run any SDK command eg. `dynamodb.list_tables()`

Search the Rust SDK for AWS_REGION and AWS_DEFAULT_REGION handling, then reproduce the issue with the shown leading-space value and a DynamoDB list_tables call. Done means the SDK reports a specific invalid-region error instead of the opaque EndpointResolutionError construction failure.

Written by the indexing model from the issue text.