Fix error handling in sendChatHistoryAsync and empty array test expectations

[Copilot]

Test failures occurred after recent refactoring: sendChatHistoryAsync threw unhandled exceptions from session.getItems() errors, and test expectations conflicted with the requirement to call the MCP platform even with empty message arrays.

Changes

• Error handling in sendChatHistoryAsync: Wrapped session.getItems() in try-catch to convert session errors to OperationResult.failed while allowing validation errors from downstream calls to propagate
• Test expectations: Updated "should return success for empty session" to expect HTTP call with empty chatHistory array, aligning with MCP platform requirements
• Type safety: Changed catch block error type from any to unknown with proper OperationError wrapping

let items: AgentInputItem[];
try {
  items = await session.getItems(limit);
} catch (err: unknown) {
  const error = err as Error;
  return OperationResult.failed(new OperationError(error));
}

// Validation errors from this method will propagate
return await this.sendChatHistoryMessagesAsync(turnContext, items, toolOptions);

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