[google_sign_in] PR 4/4 Convert the Pigeon host API from Objective-C to Swift

[victogomez-cs]

Converts the Pigeon host API from Objective-C to Swift (swiftOut / messages.g.swift) and implements GoogleSignInApi with throws/Result instead of Obj-C error pointers.

Generated Swift is committed codegen from pigeons/messages.dart (Pigeon 26.3.4), then swift-format. messages.g.h / messages.g.m are removed. The Obj-C SPM target’s publicHeadersPath is narrowed to the remaining Obj-C headers.

Missing-presenter failures from PR 3/4 are returned as PigeonError. Tests updated for the Swift Pigeon types.

Bumps google_sign_in_ios to 6.3.5.

PR 4/4 of the Obj-C → Swift migration. Depends on PR 3/4 (ViewProvider / GID wrappers). Completes flutter/flutter#119103 for the iOS/macOS plugin implementation.

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[gemini-code-assist]

Code Review

This pull request migrates the Pigeon host API for the iOS Google Sign-In plugin from Objective-C to Swift, replacing the autogenerated Objective-C files with Swift files and updating the plugin implementation and tests accordingly. Feedback on the changes suggests using the any keyword for existential protocol types to align with modern Swift standards, replacing an Objective-C style string formatting initializer with a simple string literal, and avoiding force-casting in tests to prevent potential test runner crashes.

[gemini-code-assist]

To maintain consistency with the rest of the codebase (such as GoogleSignInTests.swift) and ensure readiness for Swift 6's strict existential type requirements, please use the any keyword when referring to the existential protocol types GIDSignInProtocol and GIDGoogleUserProtocol.

Suggested change

	let signIn: GIDSignInProtocol
	/// A mapping of user IDs to GIDGoogleUser instances to use for follow-up calls.
	var usersByIdentifier: [String: any GIDGoogleUserProtocol] = [:]
	var usersByIdentifier: [String: GIDGoogleUserProtocol] = [:]
	let signIn: any GIDSignInProtocol
	/// A mapping of user IDs to GIDGoogleUser instances to use for follow-up calls.
	var usersByIdentifier: [String: any GIDGoogleUserProtocol] = [:]

References

1. Code should follow the relevant style guides and language idioms for Swift, which includes using existential any for protocol types to align with modern Swift standards and prepare for Swift 6. ^(link)

[victogomez-cs]

Done. Marked the GIDSignInProtocol / GIDGoogleUserProtocol existentials with any on the stored properties and the matching init and method parameters

[gemini-code-assist]

In Swift, we can use a simple string literal "(null): 0" instead of the Objective-C style String(format:) initializer for better readability and performance.

Suggested change

	code: String(format: "%@: %ld", "(null)", 0),
	code: "(null): 0",

[victogomez-cs]

Agreed, those format arguments were constants, so the literal is equivalent. Updated to "(null): 0"

[gemini-code-assist]

In Swift Testing, using force-casting (as!) can cause the entire test runner to crash if the cast fails, preventing other tests from running. Instead, use try #require(error as? PigeonError) to fail the test gracefully. Additionally, update the comment to refer to PigeonError instead of FlutterError since the API has been migrated. Note that this same pattern of using as! and outdated FlutterError references occurs in several other places in this file (e.g., lines 639, 732, 755, 854, 886, 945) and should be updated similarly.

Suggested change

	// Unexpected errors, such as runtime exceptions, are returned as
	// FlutterError.
	let flutterError = error as! PigeonError
	#expect(flutterError.code == "google_sign_in")
	// Unexpected errors, such as runtime exceptions, are returned as
	// PigeonError.
	let pigeonError = try #require(error as? PigeonError)
	#expect(pigeonError.code == "google_sign_in")

[victogomez-cs]

Updated the comments to PigeonError and dropped the as! casts so a type mismatch fails the test instead of crashing the runner.

I didn’t use try #require here: these assertions sit in a non-throwing completion, so try doesn’t compile, and a thrown #require would skip confirmed() and hang the test. Used guard let … as? PigeonError so confirmed() still runs