WebBrokerAPI: Fix: Propagate header-phase SharedContext to body-phase in ProcessByChainKey (hub.go)

Summary

In event-gateway/gateway-runtime/internal/hub/hub.go, the ProcessByChainKey method builds the body-phase RequestContext via MessageToRequestContext, which creates a fresh SharedContext. As a result, metadata populated during header-phase policy execution is not visible to body-phase policies.

Expected Behavior

Body-phase policies should see metadata set by header-phase policies, consistent with the pattern used in ProcessSubscribe, ProcessInbound, etc.

Suggested Fix

if chain.RequiresRequestBody {
    reqCtx := MessageToRequestContext(msg, binding)
    reqCtx.SharedContext = reqHeaderCtx.SharedContext
    bodyResult, err := h.engine.ExecuteRequestBodyPolicies(ctx, chainKey, reqHeaderCtx.SharedContext, reqCtx)
    ...
}

References

• PR: https://github.com/wso2/api-platform/pull/1954
• Review comment: https://github.com/wso2/api-platform/pull/1954#discussion_r3239562646

Requested by @senthuran16.