Dispatches a chat completion request to the configured adapter.

Pass agent: :name to pull default options (model, temperature, adapter, etc.) from the :agents configuration. Provide tools: [...] with %Synaptic.Tools.Tool{} structs (or maps/keywords convertible via Synaptic.Tools.Tool.new/1) to enable tool-calling flows. Provide mcp: [...] with MCP server descriptors to expose remote tools and MCP resource browsing.

When stream: true is passed, the response will be streamed and PubSub events will be emitted for each chunk. Note: streaming automatically falls back to non-streaming mode when tools are provided, as OpenAI streaming doesn't support tool calling.