chore: update CI to fetch base history for flutter_plugin_tools format and apply formatting

[SelaseKay]

Description

Updates FlutterFire formatting CI to match current flutter_plugin_tools behavior and formatting output.

This PR:

• updates CI to work with the newer flutter_plugin_tools merge-base logic
• passes an explicit --base-branch for formatter diffing
• stops relying on standalone swiftformat in CI
• relies on Swift formatting handled through flutter_plugin_tools
• applies the resulting formatting updates

Background

FlutterFire CI has historically run flutter_plugin_tools format without passing an explicit base branch.

Older versions of flutter_plugin_tools determined the diff base using FETCH_HEAD, which made the behavior depend on the last fetch performed in the CI checkout. Newer versions changed that logic to resolve a merge base from an explicit branch instead. In the current implementation, GitVersionFinder defaults to main when no base branch is provided, and computes the base using git merge-base <base-branch> HEAD.

That behavior means CI now needs:

a resolvable base branch locally, and
enough history for git merge-base to succeed.
Without that, formatting jobs can fail even though the same workflow worked previously with older flutter_plugin_tools.

For reference, this behavior change is visible in git_version_finder.dart in flutter/packages: https://github.com/flutter/packages/blob/main/script/tool/lib/src/common/git_version_finder.dart

What changed in this PR

1. Make the formatter base explicit
   The CI formatter invocation now passes --base-branch=origin/main.

This avoids relying on implicit checkout state and makes the formatter’s comparison target explicit.

2. Update CI to support merge-base resolution
   The workflow now ensures the formatter has access to the branch/history it needs for git merge-base to succeed with the newer flutter_plugin_tools behavior.

3. Stop using standalone swiftformat
   Previously, FlutterFire CI separately relied on swiftformat for Swift formatting.

That is no longer necessary here. This change removes that dependency and instead relies on the Swift formatting behavior provided through flutter_plugin_tools, so formatting is handled through a single formatter path rather than split across separate tools.

4. Apply formatting changes
   Because the formatter behavior/output changes as part of this update, this PR includes the resulting formatting updates.

Related Issues

Replace this paragraph with a list of issues related to this PR from the issue database. Indicate, which of these issues are resolved or fixed by this PR. Note that you'll have to prefix the issue numbers with flutter/flutter#.

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.

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