FOUNDATIONS: EventV2Service - RetrieveEventV2CountBySignatureAsync

[cjdutoit]

Summary

Foundation layer implementation of RetrieveEventV2CountBySignatureAsync with full TDD coverage:

• Returns count of matching EventV2 records within the loop detection window
• Returns 0 if loop detection is disabled
• Validates eventV2 and config non-null
• Validates fields (EventAddressId, EventName, ContentHash, Window)
• Handles timeout, SQL, and service exceptions

Test coverage

• 9 tests: 1 logic (enabled), 1 logic (disabled), 3 validation, 2 exception, 2 edge case
• All passing

closes #546