[format_command] - fixed fallback logic within formatter command class

[Hamza-Bashir4586]

*This change enhances the FormatCommand’s Swift‐format resolution logic by introducing a third, macOS-specific lookup step. Now, if the user hasn’t provided --swift-format-path and no swift-format binary is found on the PATH, the tool will invoke:

• xcrun --find swift-format

• and, if that returns a valid path, use it to format and lint Swift files. If all three strategies fail (explicit override, PATH scan, xcrun), we print an error that also suggests installing Xcode 16+.

Key changes:

• _findValidSwiftFormat() in format_command.dart:

• Check --swift-format-path

• Scan for any swift-format on PATH

• New: On macOS, fall back to xcrun --find swift-format

• On failure, updated error message to mention Xcode 16+.

• Tests in format_command_test.dart:

Added a new test under the swift-format group that simulates macOS, forces both the explicit flag and PATH lookups to fail, stubs xcrun to return /usr/bin/swift-format, and then asserts that formatting and linting invocations use that fallback binary. This ensures full coverage of the new lookup path.

This PR resolves the following issue: #153803
*

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[stuartmorgan-g]

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