Update client implementations to pass system_message when resuming sessions

Co-authored-by: patniko <26906478+patniko@users.noreply.github.com>

Author: Copilot

dotnet/src/Client.cs

@@ -436,6 +436,7 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
             sessionId,
             config?.ReasoningEffort,
             config?.Tools?.Select(ToolDefinition.FromAIFunction).ToList(),
+            config?.SystemMessage,
             config?.Provider,
             config?.OnPermissionRequest != null ? true : null,
             config?.OnUserInputRequest != null ? true : null,
@@ -1155,6 +1156,7 @@ internal record ResumeSessionRequest(
         string SessionId,
         string? ReasoningEffort,
         List<ToolDefinition>? Tools,
+        SystemMessageConfig? SystemMessage,
         ProviderConfig? Provider,
         bool? RequestPermission,
         bool? RequestUserInput,

go/client.go

@@ -703,6 +703,10 @@ func (c *Client) ResumeSessionWithOptions(sessionID string, config *ResumeSessio
 		if config.Provider != nil {
 			params["provider"] = buildProviderParams(config.Provider)
 		}
+		// Add system message configuration
+		if config.SystemMessage != nil {
+			params["systemMessage"] = config.SystemMessage
+		}
 		// Add streaming option
 		if config.Streaming {
 			params["streaming"] = config.Streaming

nodejs/src/client.ts

@@ -546,6 +546,7 @@ export class CopilotClient {
                 description: tool.description,
                 parameters: toJsonSchema(tool.parameters),
             })),
+            systemMessage: config.systemMessage,
             provider: config.provider,
             requestPermission: !!config.onPermissionRequest,
             requestUserInput: !!config.onUserInputRequest,

python/copilot/client.py

@@ -562,6 +562,11 @@ async def resume_session(
         if tool_defs:
             payload["tools"] = tool_defs
 
+        # Add system message configuration if provided
+        system_message = cfg.get("system_message")
+        if system_message:
+            payload["systemMessage"] = system_message
+
         provider = cfg.get("provider")
         if provider:
             payload["provider"] = self._convert_provider_to_wire_format(provider)