test(coding-agent): add AgentSession test harness with faux provider
Adds a reusable test harness for AgentSession runtime testing:
- Faux stream function with declarative response sequencing (text,
tool calls, thinking, errors, custom usage, delays)
- Token-level streaming with realistic delta events (3-5 char chunks)
- One-call session factory with real in-memory dependencies
- Event capture with type-filtered access
- 14 self-tests covering text, tool calls, errors, retry, streaming
deltas, thinking, event ordering, persistence
ref #2023