Add comprehensive test coverage for SendChatHistoryAsync parameter validation

[Copilot]

The original PR #148 had insufficient test coverage for SendChatHistoryAsync - tests only validated null parameters but not the Activity property validation logic or exception handling behavior.

Changes

• Added Activity property validation tests: 3 new tests covering missing Conversation, Id, and Text properties with distinct mock scenarios
• Each test validates a specific error path:
  • MissingConversationId: Activity has valid Id/Text but null Conversation
  • MissingMessageId: Activity has valid Conversation/Text but null Id
  • MissingUserMessage: Activity has valid Conversation/Id but null Text

var activityMock = new Mock<IActivity>();
activityMock.Setup(a => a.Id).Returns((string)null!); // Missing message ID
activityMock.Setup(a => a.Text).Returns("Hello");
activityMock.Setup(a => a.Conversation).Returns(conversationAccount);

All 8 tests pass (4 existing + 3 new + 1 unrelated).

Testing Limitations

HTTP behavior testing (success/error responses, timeouts, network errors) requires integration tests - the method uses RuntimeUtility.GetDefaultHttpClient() which creates HttpClient internally, making it impractical to mock in unit tests.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.