fix: classify unstreamed sub-agent messages as thinking

Sub-agent messages bypass the streaming path (turn-delta) entirely due to
SDK batching behavior. They arrive only at turn-complete time via
FinalizeCompletedMessages. Without classification, they appear as separate
chat bubbles cluttering the transcript.

Two-pronged fix:

Sidecar: FinalizeCompletedMessages now tags messages from
_reclassifiedMessageIds with MessageKind='thinking'. This covers messages
that WERE streamed and reclassified during the turn. Added MessageKind
property to ChatMessageDto.

Main process: finalizeTurn detects unstreamed assistant messages (not in
existing session.messages) and classifies them as thinking when a visible
response was already streamed. Emits message-reclassified events so the
renderer can update incrementally, though the primary path is the
workspace:updated broadcast which already includes the correct messageKind.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Aryx.AgentHost/Services/CopilotTurnExecutionState.cs

@@ -313,6 +313,14 @@ internal sealed class CopilotTurnExecutionState
                 ActiveAgent);
         }
 
+        foreach (ChatMessageDto message in CompletedMessages)
+        {
+            if (_reclassifiedMessageIds.Contains(message.Id))
+            {
+                message.MessageKind = "thinking";
+            }
+        }
+
         return CompletedMessages;
     }