Support Node.js 26.x managed runtime

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**Tell us about your request**
Please add Node.js 26.x as a supported managed runtime in App Runner (analogous to the existing NODEJS_22 runtime). Node.js 26 was [released](https://nodejs.org/en/blog/release/v26.0.0) on 2026-05-05 as the upcoming LTS version.

**Describe alternatives you've considered**
Using Node.js 22 as the current latest supported runtime, or deploying via custom container image.

**Additional context**
- Node.js 24 tracking issue: #285
- Current App Runner Node.js runtimes: https://docs.aws.amazon.com/apprunner/latest/dg/service-source-code-nodejs-releases.html

**Attachments**
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