feat(maf): emit final response as result update

[zxyao145]

Summary

Surfaces the Codex agent's final response as a result update so consumers no longer have to reassemble streamed text to know the answer.

• RunStreamingAsync now emits a result AgentResponseUpdate immediately before the terminal update (turn.completed, turn.failed or thread error). The update is intentionally not re-saving anything to chat history.
• RunAsync appends the same final response as a result chat message, so it is part of AgentResponse.Messages.
• On failure the result carries the error message instead of the agent message (last agent_message wins on success; turn.failed / thread error message wins otherwise).
• New shared helper ThreadEventExtensions.CreateResultUpdate sets type = "result", ChatRole.Assistant, AuthorName = "codex" and the normalized model name.

Changes

• src/CodexSdk.MAF/CodexAIAgent.cs — track the final response in both the streaming and non-streaming paths and emit/append it.
• src/CodexSdk.MAF/Internal/ThreadEventExtensions.cs — add CreateResultUpdate.
• tests/CodexSdk.MAF.Tests/CodexAIAgentResultUpdateTests.cs (new) — streaming/RunAsync success and failure coverage.
• tests/CodexSdk.MAF.Tests/ThreadEventExtensionsTests.cs — unit test for CreateResultUpdate.
• tests/FakeCodexCli/Program.cs — failure injection via CODEX_TEST_FAILURE (turn, thread).
• tests/Support/FakeCodexCli.cs — expose CreateOptions() and forward the failure mode.

Verification

• dotnet test: 47 passed, 0 failed (CodexSdk.Tests 12, CodexSdk.MAF.Tests 35).
• New tests assert that on success exactly one result update is emitted, positioned directly before turn.completed, with Role = Assistant and AuthorName = codex.
• Failure paths are covered for both turn.failed ("model unavailable") and thread error ("stream disconnected"), asserting the result text equals the error message in both the streaming and RunAsync paths.