Feature request: Add nodejs26.x runtime support

Please add `nodejs26.x` as a supported runtime in SAM CLI. Node.js 26 was [released](https://nodejs.org/en/blog/release/v26.0.0) on 2026-05-05 as the upcoming LTS version.

I wish SAM CLI would support `nodejs26.x` for `sam init`, `sam build`, `sam local invoke` and `sam local start-api`.

### Proposal

Analogous to existing nodejs22.x and nodejs24.x support, add nodejs26.x as a runtime option throughout the SAM CLI.

**Prerequisites:**
- aws/aws-lambda-base-images#488
- aws/aws-sam-build-images#193

If you're also waiting for this, please 👍 this issue.

### Additional Details

Node.js 26 highlights:
- V8 engine updated to v14.6 (NODE_MODULE_VERSION 147)
- Stable TypeScript support
- Temporal API enabled by default