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

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.

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.

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

openai / openai-dotnet