fix: add History message type support and prevent Content-Length header in WebSocket upgrade

[naomi-deepgram]

Summary

• History message type: The Voice Agent API returns a "type": "History" server message that was not handled by the SDK — it would silently fall through to the Unhandled event. This PR adds full support so subscribers can receive and act on History events.
• Content-Length header: On .NET 5+, WebSocket upgrade requests now use SocketsHttpHandler via HttpMessageInvoker, which does not add a Content-Length: 0 header to the upgrade request. This header violates the WebSocket protocol and causes failures with strict proxies such as Azure API Management (APIM).

Changes

History message type ("type": "History")

• Deepgram/Models/Agent/v2/WebSocket/HistoryResponse.cs — New model with Role, Content, and FunctionCalls fields; includes a HistoryFunctionCall record
• Deepgram/Models/Agent/v2/WebSocket/AgentType.cs — Added History to the AgentType enum and History = "History" to AgentClientTypes
• Deepgram/Clients/Agent/v2/Websocket/Client.cs — Added _historyReceived event, Subscribe(EventHandler<HistoryResponse>), and case AgentType.History: in ProcessTextMessage
• Deepgram/Clients/Interfaces/v2/IAgentWebSocketClient.cs — Added Subscribe(EventHandler<HistoryResponse>) to the interface

Content-Length fix

• Deepgram/Abstractions/v2/AbstractWebSocketClient.cs — On NET5_0_OR_GREATER, ConnectAsync is called with an HttpMessageInvoker wrapping SocketsHttpHandler instead of the default handler, preventing Content-Length: 0 from being included in the WebSocket upgrade request

Test plan

• 7 new unit tests added to AgentClientTests.cs covering HistoryResponse serialisation, HistoryFunctionCall field mapping, null-field omission, enum presence, and the AgentClientTypes constant
• Full test suite passes (170/170)