fix: add newline at end of firebase_options.dart

[karnemanideep]

Description

flutterfire configure generates firebase_options.dart
without a newline at the end of the file.

This causes dart format to modify the generated file
every time it is run, resulting in:

• Unnecessary changes in git diff
• CI/CD pipeline failures when dart format check is enabled
• Confusion for developers seeing unexpected file changes

##How I Found It

Searched for writeAsStringSync across the codebase to
find where firebase_options.dart content is written.

This led to:
packages/flutterfire_cli/lib/src/firebase/firebase_dart_configuration_write.dart

Inside this file, two functions build and return the
content of firebase_options.dart as a string using
.join('\n') — which joins lines with newlines BETWEEN
them but does NOT add a newline at the END of the file.

##What I Changed

File:
packages/flutterfire_cli/lib/src/firebase/firebase_dart_configuration_write.dart

Line 180 — inside _updateExistingConfigurationFile():

// Before ❌
return formatList(fileConfigurationLines).join('\n');

// After ✅
return '${formatList(fileConfigurationLines).join('\n')}\n';

Line 187 — inside _buildConfigurationFile():

// Before ❌
return formatList(_stringBuffer.toString().split('\n')).join('\n');

// After ✅
return '${formatList(_stringBuffer.toString().split('\n')).join('\n')}\n';

The \n appended at the end ensures the generated
firebase_options.dart always ends with a newline,
satisfying dart format requirements.

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

[CLAassistant]

All committers have signed the CLA.

[karnemanideep]

I've opened a PR to fix this issue.
PR: #438

[gemini-code-assist]

Code Review

This pull request modifies the Firebase configuration writer to ensure generated files end with a trailing newline. While the change to the file configuration lines is correct, a review comment points out that the change in _buildConfigurationFile likely introduces a redundant double newline because the underlying string buffer already contains a trailing newline character.

[karnemanideep]

Hi @russellwheatley!

Applied the suggested fix from Gemini
for line 187. The CI checks are awaiting
your approval to run.

Could you please take a look when
you get a chance?