aws-runtime, aws-sigv4, aws-credential-types, and the aws-smithy-*
crates bumped their MSRV to 1.91.1 in late 2026, so they no longer
build on our pinned 1.91.0 MSRV. Pin them to the latest 1.91.0
compatible versions in the MSRV downgrade step.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>