Feature: Consolidate Agent and MCPServer Webhooks to Avoid Code Duplication

[cwiklik]

Overview

Currently, sidecar injection logic exists in two places:

• kagenti operator: Directly injected sidecars into Agent CRs during reconciliation

• kagenti-extensions webhook: Injected sidecars into MCPServer CRs via admission webhook

This duplication creates maintenance overhead, increases the risk of implementation drift, and makes it harder to ensure consistent behavior across both resource types.