fix(core): use valid-format placeholder options for demoProjectId apps

[SelaseKay]

Description

Firebase.initializeApp(demoProjectId: ...) fills in placeholder options with API key '12345' and an empty messagingSenderId. On Apple platforms, native Firebase SDKs check these values as soon as the app is configured, and they raise uncaught exceptions that abort the app:

• Firebase Installations, which firebase_crashlytics, firebase_messaging and others pull in, requires an API key of 39 URL-safe base64 characters starting with A.
• Firebase Remote Config requires a non-empty GCM sender ID.

This PR changes the placeholder API key to 'ADemoProjectApiKey000000000000000000000' and the sender ID to '1', which matches the project number in the placeholder app ID 1:1:<platform>:1. The key deliberately doesn't start with AIza, so secret scanners won't flag it as a leaked Google API key. Android's Firebase Installations uses the same key format, so both values apply on every platform. Emulators accept any API key.

Related Issues

• Fixes [firebase_core] Firebase.initializeApp with demoProjectId crashes on iOS with SIGABRT (FIRInstallations validateAPIKey) - Follow-up to locked #17788 #18790
• Follow-up to [firebase_core]: Firebase.initializeApp throws an exception when run with demoProjectId #17788

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

Test plan

• New e2e test Firebase.initializeApp() with demoProjectId in tests/integration_test/firebase_core/firebase_core_e2e_test.dart, which runs in the core shard. On an iOS simulator with the tests app, the app aborts during this test on main and passes with this change.
• flutter test test/firebase_core_test.dart in packages/firebase_core/firebase_core passes with the updated expected options.

[gemini-code-assist]

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize the Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counterproductive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for GitHub and other Google products, sign up here.