Add ability to set a runtime flag in the CLI

[falahat]

Description

Adding the ability to set a runtime version in the CLI when creating a backend.

Scenarios Tested

Note, I havent tested these yet, but it's the plan:

1. Interactive backend creation:
   • Run firebase apphosting:backends:create and confirm you are prompted to select a runtime.
2. Backend creation with runtime flag:
   • Run firebase apphosting:backends:create --runtime nodejs22 and verify the backend is created with the specified runtime.
3. Non-interactive backend creation (default runtime):
   • Run firebase apphosting:backends:create --backend --primary-region --non-interactive and verify the backend
     is created with the default nodejs runtime.
4. Non-interactive backend creation (specific runtime):
   • Run firebase apphosting:backends:create --backend --primary-region --runtime nodejs22 --non-interactive and
     verify the backend is created with the nodejs22 runtime.
5. Unit Tests:
   • The new unit tests in src/apphosting/backend.spec.ts verify that the createBackend function correctly passes the chosen
     runtime to the App Hosting API.

Sample Commands

Create a backend with a specific runtime:

firebase apphosting:backends:create --runtime nodejs22

Create a backend non-interactively (defaults to nodejs runtime):

firebase apphosting:backends:create --backend my-backend --primary-region us-central1 --non-interactive

Create a backend non-interactively with a specific runtime:

firebase apphosting:backends:create --backend my-backend --primary-region us-central1 --runtime nodejs22 --non-interactive

Prompt for runtime selection in interactive mode:

firebase apphosting:backends:create
# Expected Prompt:
# ? Which runtime do you want to use? (Use arrow keys)
# ❯ Node.js (default)
#   Node.js 22

Today, the list of valid runtimes is hardcoded but the CLI will be able to call the Control Plane API, ListSupportedRuntimes, to fetch the list live.

Create a backend with an empty runtime (defaults to nodejs runtime):

firebase apphosting:backends:create --backend my-backend --primary-region us-central1 --runtime "" --non-interactive
# Expected Result: Backend created with 'nodejs' runtime.

Create a backend with an unsupported runtime (expected to fail):

firebase apphosting:backends:create --runtime nodejs18
# Expected Error:
# Error: HTTP Error: 400, InvalidArgument: runtime "nodejs18" is not supported

[gemini-code-assist]

Summary of Changes

Hello @falahat, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Firebase CLI's apphosting:backends:create command by enabling users to explicitly define the runtime environment for their new App Hosting backends. It provides flexibility through a new command-line flag and an interactive selection process, while also ensuring a sensible default for non-interactive operations. The changes integrate this new capability into the backend creation logic and include corresponding unit tests.

Highlights

• New CLI Option: Introduced a new --runtime option for the firebase apphosting:backends:create command, allowing users to specify the desired runtime (e.g., nodejs, nodejs22) when creating a backend.
• Interactive Runtime Selection: When creating a backend interactively and no runtime is specified, the CLI will now prompt the user to select between available runtimes, such as 'Node.js (default)' and 'Node.js 22'.
• Non-Interactive Default Runtime: For non-interactive backend creation, if the --runtime flag is not provided, the system will automatically default to the nodejs runtime.
• Backend API Integration: The createBackend function and related API interfaces have been updated to accept and process the specified runtime, ensuring it is correctly passed to the App Hosting API.
• Unit Test Coverage: New unit tests have been added to verify that the createBackend function correctly handles and passes both nodejs and nodejs22 runtimes to the underlying API.

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[gemini-code-assist]

Code Review

This pull request adds the ability to specify a runtime via a new --runtime flag in the CLI when creating an App Hosting backend. The implementation is straightforward and includes new unit tests to cover the changes. My review includes a couple of suggestions to improve code maintainability by reducing duplication in both the core logic and the tests.