[OTA SU] Use run_if_endpoint_matches instead of fixed EP0 (TC-SU-2.2 feedback)

[juandediosg]

Summary

Following the reviewer feedback on test TC-SU-2.2: Automate regarding the use of a hardcoded endpoint=0, the subscription logic must be updated to ensure the Requestor cluster endpoint is explicitly validated instead of assumed.

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Endpoint Handling Updates

• Replace the fixed endpoint=0 in AttributeSubscriptionHandler.start() with logic that determines the correct endpoint for the OtaSoftwareUpdateRequestor cluster.
• Integrate run_if_endpoint_matches to enforce explicit endpoint matching and avoid relying on endpoint 0 assumptions.