fix!: eliminate MessageId collisions in dialogue routing

[YuKitsune]

Replies were landing in the wrong dialogue because MessageId is not unique across dialogues, uplinks, or downlinks, making the search in FindDialogueForMessage unreliable. Replaced the ambiguous search with four explicit paths - controller uplinks now route via BeginDialogueCommand or ReplyToDownlinkCommand (using DialogueId directly), and incoming downlinks route via dedicated notifications that either create a new dialogue or search open dialogues by uplink message ID only.

Closes #34

[YuKitsune]

Code review

Found 1 issue:

1. SendUplinkCommandHandler now unconditionally creates a new Dialogue (even when ReplyToDownlinkId is set), but LogonCommandHandler still calls SendUplinkCommand with ReplyToDownlinkId: request.DownlinkId on all three paths. The LOGON downlink is added to a dialogue by DownlinkReceivedNotificationHandler, then the LOGON ACCEPTED / LOGON REJECTED uplink lands in a separate orphan dialogue instead of being appended to it. The previous append-on-reference branch was removed and the regression is locked in by the renamed test Handle_AlwaysCreatesNewDialogue_EvenWithReference.

CPDLCPlugin/source/CPDLCServer/Handlers/SendUplinkCommandHandler.cs

Lines 40 to 46 in da4acf5

	var dialogue = new Dialogue(request.Recipient);
	var uplinkMessage = dialogue.AddUplink(
	messageId,
	request.ReplyToDownlinkId,
	request.Recipient,
	request.Sender,

CPDLCPlugin/source/CPDLCServer/Handlers/LogonCommandHandler.cs

Lines 36 to 42 in da4acf5

	new SendUplinkCommand(
	"SYSTEM",
	request.Callsign,
	request.DownlinkId,
	CpdlcUplinkResponseType.NoResponse,
	"LOGON ACCEPTED"),
	cancellationToken);

CPDLCPlugin/source/CPDLCServer/Handlers/LogonCommandHandler.cs

Lines 66 to 73 in da4acf5

	await mediator.Send(
	new SendUplinkCommand(
	"SYSTEM",
	request.Callsign,
	ReplyToDownlinkId: request.DownlinkId,
	CpdlcUplinkResponseType.NoResponse,
	"LOGON REJECTED. NO ATS AVBL."),
	cancellationToken);

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