[tool] Use swift-format from Xcode

[stuartmorgan-g]

As of Xcode 16, swift-format is part of the Xcode distribution. Since CI now uses Xcode 16, we can just always use that version.

This:

• Removes the swift-format-path logic.
• Removes the check for swift-format being available
• Defaults Swift formatting to true only on macOS
• Removes the swift-format CIPD package from .ci.yaml
• Updates Pigeon in packages using Swift Pigeon generation to pick up a fix for a lint issue picked up by the latest version of swift-format (which was already fixed in Pigeon).

Fixes flutter/flutter#153803

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[stuartmorgan-g]

@jmagman / @vashworth Any objection to this approach of just unconditionally using xcrun, rather than using the multi-step check originally described in flutter/flutter#153803 ?

We could still do that version, if we wanted to support overriding the swift-format version (or to allow running swift-format from other platforms), but in practice I think that now that it's part of Xcode the chances that anyone will ever actually want to run a version that's not the Xcode version is very low, so probably not worth the complexity in the tool.

[stuartmorgan-g]

This change preserves the old behavior. When I first wrote this I used inheritance out of habit, but that didn't actually do anything in Pigeon at the time. I fixed it by adding composition (line 62 below), but forgot to remove this extends that was being ignored.

In the current Pigeon this is an error, because inheritance does work, but only to inherit from a sealed empty base class to allow type switching.