fix: guard FlutterTaskQueue for backward compatibility

[muzahidul-opti]

Summary

• Guards makeBackgroundTaskQueue call with responds(to:) in SwiftOptimizelyFlutterSdkPlugin.register(with:) to support older Flutter versions (< 3.22) where FlutterTaskQueue protocol is incomplete
• Falls back to creating the logger FlutterMethodChannel without a taskQueue when the API is unavailable
• Adds spec document at specs/002-flutter-task-queue-compat/spec.md

Context

The testapp iOS CI uses Flutter 3.16.0 with Xcode 26.6. At this combination, the Swift compiler cannot import makeBackgroundTaskQueue because the FlutterTaskQueue protocol is forward-declared but not fully defined in the ObjC header. This causes two hard compilation errors:

error: value of type 'any FlutterBinaryMessenger' has no member 'makeBackgroundTaskQueue'
error: extra argument 'taskQueue' in call

The taskQueue parameter is an optimization (offloads logger callbacks from the main thread). Passing nil falls back to the default behavior with no functional regression.

Test plan

• Testapp iOS CI passes with Flutter 3.16.0 + Xcode 26.6
• Local build succeeds with Flutter 3.44.0 + Xcode 26.6 (taskQueue path taken)
• flutter test passes (no Dart changes)

🤖 Generated with Claude Code

[muzahidul-opti]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merged master into this branch — no conflicts, the merge applied cleanly.

[muzahidul-opti]

@copilot review the pr

[Copilot]

@copilot review the pr

Review complete. The Swift fix is correct — makeBackgroundTaskQueue/taskQueue are fully removed from logger channel creation, and DispatchQueue.main.async in OptimizelyFlutterLogger already handles thread safety with no regression.

One spec doc inaccuracy was found and fixed (commit $(git rev-parse --short HEAD)): the "Solution" section was still describing the abandoned responds(to:) runtime-guard approach rather than the actual implementation. The spec now accurately documents dropping taskQueue entirely and the rationale for doing so.

[pvcraven]

Removes logger/background task for backwards compatability.