[google_sign_in] Increase iOS coverage tests

[victogomez-cs]

Adds native unit tests for previously untested paths in FLTGoogleSignInPlugin.m before the Objective-C → Swift migration. Production code is unchanged.

This is a tests-only change, so it does not bump the package version or CHANGELOG.

Native unit tests go from 21 tests / 6 suites to 31 tests / 11 suites.

New test cases

urlHandling (iOS / Mac Catalyst only) — these APIs do not exist on macOS (handleOpenURLs: is the macOS path):

• applicationOpenURL — application:openURL:options: forwards the URL to GID and returns true
• applicationOpenURLReturnsHandleResult — same path returns false when GID does
• sceneOpenURLContexts — scene:openURLContexts: forwards the URL from a UIOpenURLContext stand-in

errorMapping:

• mapsRemainingGIDSignInErrorCodes (parameterized):
  • GIDSignInError.keychain → FSIGoogleSignInErrorCode.keychainError
  • GIDSignInError.EMM → FSIGoogleSignInErrorCode.eemError
  • GIDSignInError.unknown → FSIGoogleSignInErrorCode.unknown
  • unrecognized SDK code (12345) → FSIGoogleSignInErrorCode.unknown
• sanitizesComplexUserInfoInFlutterError — FSISanitizedUserInfo keeps strings/numbers/URLs/arrays/dicts/nested errors and stringifies unsupported types (e.g. Date)

disconnect:

• disconnectReturnsFlutterErrorOnFailure — a GID disconnect failure is returned as a Flutter error with domain/code and userInfo

userData:

• signInWithoutProfileImageOmitsPhotoUrl — a user with no profile photo gets photoUrl == nil and still maps displayName

topViewController (iOS / Mac Catalyst only) — macOS presents with NSWindow, not a UIViewController hierarchy:

• usesNavigationControllerVisibleController — presents from the nav stack’s top VC
• usesTabBarControllerSelectedController — presents from the selected tab VC
• usesPresentedViewController — presents from a presented VC

Also records URLs passed to TestSignIn.handle(_:) so the URL-handling cases can assert the fake was called.

First part of Swift migration to backfill code coverage for flutter/flutter#119103

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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 followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception 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.

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[gemini-code-assist]

Code Review

This pull request adds a comprehensive suite of Swift unit tests for the iOS Google Sign-In plugin, covering URL handling, error mapping, disconnect scenarios, user data parsing, and top view controller resolution. The review feedback suggests passing nil instead of a scene instance in the sceneOpenURLContexts test to prevent potential test failures in headless CI environments where connected scenes may be empty.

[gemini-code-assist]

In headless unit test environments (such as CI runners), UIApplication.shared.connectedScenes can be empty, which causes try #require(UIApplication.shared.connectedScenes.first) to throw and fail the test. Since the scene parameter is completely unused in the plugin's implementation of scene:openURLContexts:, we can pass nil instead and make the test more robust and independent of the application's scene state.

      @Test func sceneOpenURLContexts() {
        let (plugin, fakeSignIn) = createTestPlugin()
        let url = URL(string: "com.googleusercontent.apps.test:/oauthredirect")!
        let fakeContext = FakeOpenURLContext(url: url)

        plugin.perform(
          NSSelectorFromString("scene:openURLContexts:"),
          with: nil,
          with: NSSet(object: fakeContext))

        #expect(fakeSignIn.handledURLs == [url])
      }