Add support for retrieving File Search result content in Run Steps and other fixes

[joseharriaga]

Features added

• OpenAI.Assistants:
  • Added a Content property to RunStepFileSearchResult (step_details.tool_calls.file_search.results.content in the REST API).
    • When using an Assistant with the File Search tool, you can use this property to retrieve the contents of the File Search results that were used by the model.
  • Added FileSearchRankingOptions and FileSearchResults properties to RunStepDetailsUpdate.

Breaking Changes in Preview APIs

• OpenAI.RealtimeConversation:
  • Renamed the From*() factory methods on ConversationContentPart to Create*Part() for consistency.
  • Removed an extraneous toolCallId parameter from ConversationItem.CreateSystemMessage().
• OpenAI.Assistants:
  • Renamed RunStepType to RunStepKind.
  • Changed RunStepKind from an "extensible enum" to a regular enum.
  • Renamed the ToolCallId property of RunStepToolCall to Id.
  • Renamed the ToolKind property of RunStepToolCall to Kind.
  • Replaced the FileSearchRanker and FileSearchScoreThreshold properties of RunStepToolCall with a new FileSearchRankingOptions property that contains both values to make it clearer how they are related.

Bugs fixed

• OpenAI.RealtimeConversation:
  • Fixed serialization issues with ConversationItem creation of system and assistant messages.