[IzaakGough]

Fixes #11393.

Deploying with Node.js 26 failed with "not a supported version", even though Cloud Run functions supports it in Preview.

This adds nodejs26 as a beta runtime, the same way Node.js 22 and 24 were added.

• firebase init functions stays on Node.js 24. It picks the newest GA runtime, so new projects are not put on a Preview one. Dart has no GA runtime, so it still gets the newest.
• Deprecation dates are not published yet. They follow Google's usual rule (Node.js end of life, then six months later), which the published Node.js 22 and 24 dates match.

Tested with unit tests and a real 2nd gen deploy using both engines.node and runtime. 1st gen not tested.

[gemini-code-assist]

Code Review

This pull request adds support for the Node.js 26 runtime (Preview) for Cloud Functions, updating the configuration schema, changelog, and runtime definitions. It also modifies the runtime selection logic to prefer GA runtimes over preview/beta runtimes to prevent scaffolding new projects on preview runtimes, falling back to the newest runtime if no GA runtime exists. Feedback on the changes suggests defensively guarding the runtime status check with optional chaining (RUNTIMES[r]?.status) to handle potentially undefined runtimes safely and align with the repository's strict null check guidelines.