Python Server SDK: docstring pass A, agent core (AgentBase, mixins, SWMLService, FunctionResult, contexts, DataMap)

[hey-august]

Repo: signalwire-python. Files: core/agent_base.py, core/mixins/*, core/swml_service.py, core/function_result.py, core/contexts.py, core/data_map.py, agent_server.py.

• Google style: summary, Args, Returns, Raises, Example on entry points. Port prose and examples from the Fern pages under fern/products/server-sdks/pages/reference/python/agents/ before the Fern retirement issue removes them.

Done when: every public method in these files has Args/Returns where applicable; lint clean.

Part of #724