add validation for the app directory prompt

[aalej]

Description

When prompted for the app hosting app directory and you provide a non-existent path the CLI will error out:

=== App Hosting Setup
i  This command links your local project to Firebase App Hosting. You will be able to deploy your web app with `firebase deploy` after setup.
✔ Please select an option Link to an existing backend
✔ Which backend would you like to link? BACKEND_ID
i  === Deploy local source setup
✔ Specify your app's root directory relative to your firebase.json directory /non-existent
✔  Wrote configuration info to firebase.json
i  Writing default settings to apphosting.yaml...

Error: Failed to write apphosting.yaml file because app root directory /Users/PATH/apphosting-folder-setup/non-existent does not exist. Please try again with a valid directory.

Updated the behavior to validate the path during prompting:

=== App Hosting Setup
i  This command links your local project to Firebase App Hosting. You will be able to deploy your web app with `firebase deploy` after setup.
✔ Please select an option Link to an existing backend
✔ Which backend would you like to link? BACKEND_ID
i  === Deploy local source setup
? Specify your app's root directory relative to your firebase.json directory /non-existent
> Directory /Users/PATH/apphosting-folder-setup/non-existent does not exist. Please enter a valid directory.

Scenarios Tested

$ firebase init apphosting 

     ######## #### ########  ######## ########     ###     ######  ########
     ##        ##  ##     ## ##       ##     ##  ##   ##  ##       ##
     ######    ##  ########  ######   ########  #########  ######  ######
     ##        ##  ##    ##  ##       ##     ## ##     ##       ## ##
     ##       #### ##     ## ######## ########  ##     ##  ######  ########

You're about to initialize a Firebase project in this directory:

  /Users/PATH/apphosting-folder-setup

Before we get started, keep in mind:

  * You are initializing within an existing Firebase project directory


=== Project Setup

First, let's associate this project directory with a Firebase project.
You can create multiple project aliases by running firebase use --add, 

i  Using project PROJECT_ID (PROJECT_NAME) .

=== App Hosting Setup
i  This command links your local project to Firebase App Hosting. You will be able to deploy your web app with `firebase deploy` after setup.
✔ Please select an option Link to an existing backend
✔ Which backend would you like to link? BACKEND_ID
i  === Deploy local source setup
? Specify your app's root directory relative to your firebase.json directory /non-existent
> Directory /Users/PATH/apphosting-folder-setup/non-existent does not exist. Please enter a valid directory.

Sample Commands

[gemini-code-assist]

Code Review

This pull request adds input validation to ensure the specified App Hosting root directory exists. The reviewer noted that the path resolution should use config.path(input) instead of process.cwd() to correctly resolve relative to the firebase.json directory, and suggested removing the unnecessary async keyword.

[gemini-code-assist]

The prompt asks for the app's root directory relative to the firebase.json directory. However, the validation logic resolves the path using process.cwd(), which may differ from the directory containing firebase.json (e.g., if the CLI is run from a subdirectory).

To ensure correctness, resolve the path using config.path(input) instead of path.join(process.cwd(), input). Additionally, since the validation function does not perform any asynchronous operations, the async keyword can be removed.

Suggested change

	validate: async (input: string) => {
	const absPath = path.join(process.cwd(), input);
	if (!existsSync(absPath)) {
	return `Directory ${absPath} does not exist. Please enter a valid directory.`;
	}
	return true;
	},
	validate: (input: string) => {
	const absPath = config.path(input);
	if (!existsSync(absPath)) {
	return `Directory ${absPath} does not exist. Please enter a valid directory.`;
	}
	return true;
	},

[aalej]

the absPath code was copied from over

firebase-tools/src/init/features/apphosting.ts

Line 122 in 073a953

const absRootDir = path.join(process.cwd(), backendConfig.rootDir);

Although, aynsc should be removed since there are no promises or awaits inside the function. Good call out!

[joehan]

This logic assumes that process.cwd() == the firebase directory. However, if you run init from a child of the firebase directory, that will not be true. We've got some utils that resolve the firebase project dir - lets use those instead here.

[aalej]

Good catch! Running the init command on a sub-directory does break the prompt. We do set a config early on in

firebase-tools/src/config.ts

Line 52 in 2e3b10d

this.projectDir = this.options.projectDir || detectProjectRoot(this.options);

switching process.cwd() to config.projectDir

[wiz-9635d3485b]

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Scanner	Findings
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