[Feature Request] Add nodejs26.x runtime support to SAM specification

Node.js 26 was released on 2026-05-05 as the upcoming LTS version. The SAM transform / specification needs to recognize `nodejs26.x` as a valid Lambda runtime value so that SAM templates using the new runtime are accepted and correctly transformed into CloudFormation templates.

**Requested changes**
- Add `nodejs26.x` to the list of supported Lambda runtimes in the SAM specification schema and transform validator
- Update integration-test templates (e.g. in `integration/resources/templates/`) that reference Node.js runtimes to
include or cover nodejs26.x
- Update any documentation examples that reference the latest Node.js runtime

**Relationship to other SAM components**
This is part of a chain of SAM-related issues for nodejs26.x support:
- aws/aws-sam-build-images#193 (prerequisite)
- aws/aws-sam-cli#8953