Support Immediate Streaming for Server-Side Tool Invocations

### Checked other resources - [x] This is a feature request, not a bug report or usage question. - [x] I added a clear and descriptive title that summarizes the feature request. - [x] I used the GitHub search to find a similar feature request and didn't find it. - [x] I checked the LangChain documentation and API reference to see if this feature already exists. - [x] This is not related to the langchain-community package. ### Feature Description I would like LangChain to support **immediate streaming emission of server-side tool calls** when using OpenAI's server-side tools (`web_search_preview`, `code_interpreter`, `file_search`, etc.) with the Responses API. Currently, when streaming with server-side tools, the `server_tool_call` content block is not emitted when the model decides to call the tool. Instead, it's buffered and emitted together with `server_tool_result` after the tool execution completes. **What I'm requesting:** This feature would allow LangChain to emit server-side tool calls immediately (when execution begins) by handling OpenAI's `.in_progress` status events, similar to how `function_call` and `reasoning` blocks already emit on `.added` events. **Specific functionality:** - Emit `server_tool_call` content blocks when tool execution begins (via `.in_progress` status events) - Emit `server_tool_result` content blocks when tool execution completes (via `.done` events) - Provide proper separation between tool call and result in the streaming timeline - Match the existing streaming pattern for `function_call` and `reasoning` blocks ### Use Case I'm building a RAG application with web search integration and code execution capabilities. My users need real-time feedback about what the AI agent is doing during streaming responses. **Current problem:** When the model decides to search the web or execute code, there's ~3 seconds with no feedback to the user. Then once the tool is finished executing it sends both the `server_tool_call` AND the `server_tool_r…