fix(ec2): FindConvertibleOffering reads one offerings page, so a later-page match is reported as none #426

Summary

FindConvertibleOffering (providers/aws/services/ec2/client.go:859, main 740dfbd) reads a single DescribeReservedInstancesOfferings page (MaxResults: 20) and takes a match from it. The request uses the Filters style that the code comments tie to empty pages that still carry a NextToken. So a valid convertible offering on a later page is reported as "no convertible offering".

Impact

It fails loud, never picks a wrong offering, but a valid RI exchange can be refused. #310 adds an Availability Zone filter and an in-zone check to this lookup and keeps the single page, so this stays open after #310.

Expected

Walk NextToken with a bounded page cap, as ListTargetOfferings does (client.go:994), until a page yields a matching offering. Hitting the cap is an error, not a silent "none".

Test

A fake returning an empty first page with a NextToken and the match on page 2 finds the offering; a fake past the cap errors.

Refs #310, #233.