Annotate Checkout Element presentation methods with MainThread

[cttsai-stripe]

Summary

Annotate Checkout Element presentation methods with @MainThread.

Motivation

PaymentElement.present() and ShippingAddressElement.present() synchronously access UI, lifecycle, Activity Result, and state-holder behavior. This makes the existing caller requirement explicit for static analysis and IDE tooling with AndroidX @MainThread, without changing runtime behavior.

Testing

• Added tests
• Modified tests
• Manually verified

Existing focused PaymentElementTest and ShippingAddressElementTest passed. :paymentsheet:apiDump, :paymentsheet:apiCheck, :paymentsheet:detekt, and git diff --check passed. The push hook also reran :paymentsheet:apiCheck and :paymentsheet:detekt successfully.

Screenshots

Not applicable. This is an annotation-only API contract change with no visual or runtime UI change.

Changelog

No changelog entry. This preview API annotation documents an existing caller contract and does not change runtime behavior.

Committed and created by Codex.

[cttsai-stripe]

SavedStateHandle.set/get is @MainThread

[cttsai-stripe]

We could mark out internal implementations as well as a reminder for ourselves.

[jaynewstrom-stripe]

This feels like we're leaking implementation details. Can we coroutineScope + Dispatchers.Main.immediate these instead?

[cttsai-stripe]

This feels like we're leaking implementation details. Can we coroutineScope + Dispatchers.Main.immediate these instead?

IIUC, if we do that and present() is called by a background thread, the action would be posted to the main thread, and would not block the thread synchronously. It's a little bit hard to imagine what would happen if a main thread + a background thread call present() at the same time so I chose the annotation to prevent integration issues.