fix(schematics): read the signed-in Firebase account on firebase-tools 15.26+

[armando-navarro]

Fixes #3768

firebase-tools 15.26 switches to non-interactive mode when it detects an AI agent, and in that mode login() returns undefined instead of the signed-in account. ng add and ng deploy read .email from that result and crash.

Changes

• A new getActiveAccount helper in src/schematics/firebaseTools.ts passes interactive: true to login(). With an account signed in, login() returns it before any prompt, on firebase-tools 14 and 15.
• Setup reads the active account through that helper in both places it needs it.
  • It no longer passes its own options to login(). Those options carry the Angular project name, which firebase-tools checks as a Firebase project ID, so a project named myApp stopped setup with Invalid project id: myApp. on any firebase-tools version.
• The firebase login and firebase login:add commands setup starts get --interactive, so under an agent they sign the person in instead of printing remote sign-in steps.
• When firebase login leaves no account signed in, setup says to run firebase login instead of crashing.
• ng deploy signs in only when no account is listed, stops with the same kind of message if none is afterward, and reads the account through the helper.
• login() is typed as possibly undefined, so the compiler flags an unguarded .email read.
• The Quickstart and the firebase-tools install page stop saying to install firebase-tools 14. They say to run ng add yourself rather than through an AI agent, and to run it again if it already stopped.

Verification

• ng add setup against a real Firebase project reached the account, project and app questions with:
  • firebase-tools 15.28.1 with AI_AGENT and CLAUDECODE set, which crashed before this change
  • firebase-tools 15.28.1 without them
  • firebase-tools 14.27.0
• With --project=myApp, setup now gets past the account question, where it previously stopped with Invalid project id: myApp.
• With what ng add @angular/fire@next installs today (21.0.0-rc.1-canary.95b3de1, built from the same commit as 21.0.0-rc.1), after a run that crashed under an agent, running ng add again without the agent variables completed setup. The crashed run left only an empty firebase.json.
• Three new ng deploy specs cover the interactive flag, an undefined result for a signed-in account, and the no-account message. npm run test:node after a full build: 334 specs, 0 failures.
• userPrompt has no spec, because the spec runner loads files as ES modules and prompts.ts calls require() when it loads.