[Local Catalog] Fix flaky and slow POS catalog sync tests

[jaclync]

For WOOMOB-1441
Just one review is required.

Description

Fixes flaky test failures and slow test execution in POS catalog sync tests caused by concurrent access to shared state and retry delay logic. I was able to reproduce the flakiness and crash locally by running the test suite repeatedly.

Issues Fixed

1. Flaky test crashes from concurrent counter updates: Tests were randomly failing or crashing when running in parallel due to concurrent updates to integer counters in local variables and MockPOSCatalogSyncRemote
2. Slow test execution (14s delay): The startFullSync_propagates_network_errors and startIncrementalSync_throws_error_when_product_loading_fails tests were taking 14 seconds to complete due to exponential backoff retry delays

Changes

• Introduced thread-safe Counter actor for tracking call counts in test mocks and assertions
• Updated MockPOSCatalogSyncRemote to use Counter instead of plain integers for all call count properties
• Updated BatchedRequestLoaderTests to use Counter for thread-safe attempt tracking
• Added retryDelay parameter to sync service initializers to allow configuring retry delays (defaults to 2.0s for production)
• Modified error propagation tests to use retryDelay: 0 to eliminate unnecessary delays in test execution

Steps to reproduce

Run the following test suites repeatedly locally to verify that each test suite completes almost right away without crashing:

• BatchedRequestLoaderTests
• POSCatalogFullSyncServiceTests
• POSCatalogIncrementalSyncServiceTests

Testing information

I tested the above in Xcode 26 with iPad A16 iOS 18.4 simulator.

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	pr16227-ef47178
	Version	23.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	ef47178
	Installation URL	7l8pub1j0vojg

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[staskus]

👍 Appreciate the optimization. Looks good to me, good use of the actor in the counter.