Support dynamic tool addition/removal after agent creation and in middleware

### 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 Currently, all tools must be registered when calling `create_agent()`, and middleware can only filter existing tools. There's no way to dynamically add or remove tools after agent creation or within middleware. This creates challenges in scenarios like: 1. **Model Context Protocol (MCP)**: MCP servers can dynamically expose new tools based on user authentication or context changes 2. **Agent UI (ag-ui)**: Interactive applications may need to add/remove tools based on user actions or runtime conditions **Current Limitation**: Middleware can only filter pre-registered tools. Tools that are discovered at runtime (e.g., from MCP servers, plugins) cannot be added. ### Use Case - MCP servers discovering tools based on user permissions - Plugin systems loading tools from extensions - Multi-tenant systems with user-specific tools - Context-aware tool availability ### Proposed Solution 1. **Add API for dynamic tool management**: ```python agent = create_agent(model="gpt-4o", tools=[tool1, tool2]) # Add/remove tools after creation agent.add_tool(tool3) agent.remove_tool(tool1) agent.add_tools([tool4, tool5]) ``` 2. **Allow middleware to add/remove tools, not just filter**: ```python @wrap_model_call def dynamic_tool_manager(request, handler): # Add new tools dynamically new_tools = load_tools_from_mcp_server(request.state) request = request.override(tools=request.tools + new_tools) # Or remove specific tools filtered_tools = [t for t in request.tools if t.name != "restricted_tool"] request = request.overri…