[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