Support Environment Variables for Local Builds

[falahat]

Description

Adds support for Environment Variables (from apphosting.yaml files, for example) and AutoInit Env Vars for Local Builds.
This correctly splits Build vs Runtime env vars. It passes build env vars to the build process and passes runtime env vars to the control plane when creating the build. It injects the SDK AutoInit env vars into both.

Scenarios Tested

• I created a local build backend with ABIU enabled and the nodejs22 runtime
• I set set the localBuild flag to true in firebase.json:

{
  "apphosting": {
    "backendId": "local-build",
    "rootDir": "/",
    "localBuild": true,
    "ignore": [
      "node_modules",
      ".git",
      "firebase-debug.log",
      "firebase-debug.*.log",
      "functions"
    ]
  }
}

• I ran firebase deploy
• I verified the app was built locally and deployed as expected

Sample Commands

[gemini-code-assist]

Code Review

This pull request introduces logic to split environment variables into build and runtime categories and injects Firebase configuration (autoinit variables) into the local build process for App Hosting backends. Key changes include the addition of the splitEnvVars utility, updates to localBuild to handle process environment injection and restoration, and logic in the prepare phase to fetch and merge web app configurations. I have no feedback to provide as the existing review comment was purely evaluative and did not point out an issue or improvement opportunity.

[falahat]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces functionality to split environment variables into build and runtime categories and injects Firebase SDK auto-init variables during local builds. Feedback focuses on improving the robustness of error handling when loading configurations and refining type definitions for environment variable values to avoid unnecessary type assertions in tests.

[falahat]

I have verified this one last time after our fixes.