feat(maf): include model name in response updates

[zxyao145]

Summary

• ThreadEventExtensions.ToAgentResponseUpdate now accepts an optional model name and stamps it into AgentResponseUpdate.AdditionalProperties["modelName"] (trimmed; empty string when not configured).
• CodexAIAgent passes the configured ThreadOptions.Model through in both the non-streaming (RunAsync) and streaming (RunStreamingAsync) paths.
• AuthorName is now always set to codex (previously only when empty), and the redundant agentName entry in AdditionalProperties was removed from all thread-event update factories.
• ToChatMessage falls back to the codex agent name when the source update has no author.

Tests

• Updated ThreadEventExtensionsTests assertions for the new AuthorName/modelName behavior and the removal of the agentName property.
• Added a theory test covering model-name mapping (trim, null, and whitespace inputs) and a test verifying ToChatMessage falls back to the agent name when the update author is missing.

Note: test execution results are not included here; the test suite was not run as part of creating this PR.