Fix github enterprise integration config silo error

[trillville]

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Fixes `InitializationError: Error getting github enterprise integration config` caused by a silo boundary violation.

Previously, the `SeerRpcServiceEndpoint` (running in REGION silo mode) attempted to directly access the CONTROL-plane `Integration` model. This PR modifies the `IntegrationService` to be silo-aware, ensuring that when operating in a REGION silo, it uses the hybrid cloud integration service for cross-silo communication to retrieve `Integration` details. The `SeerRpcServiceEndpoint` is updated to utilize this silo-aware service.

This change prevents direct CONTROL-plane model access from REGION silos, resolving the `400 Bad Request` and subsequent `InitializationError`.

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Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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