Extract the verbatim-duplicated SiteCallOperationalDto -> SiteCall mapper
into a single public SiteCallDtoMapper static class in
ScadaLink.Communication.Grpc, mirroring AuditEventDtoMapper. Replaces three
identical private copies (SiteStreamGrpcServer.MapSiteCallFromDto,
ClusterClientSiteAuditClient.MapSiteCall, and the test-infra
DirectActorSiteStreamAuditClient.MapSiteCallFromDto), removes the now-stale
doc comment that justified the duplication, and drops the using directives
that became unused. Adds SiteCallDtoMapperTests for field-by-field coverage.
Only the FromDto direction is provided: nothing maps SiteCall back onto the
wire, so a ToDto would be dead code.
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