test: migrate processor tests to HTTP mock LLM server

Replace the custom TestLLM Effect service with the real LLM layer +
TestLLMServer HTTP mock for 9 of 10 processor tests. Tests now exercise
the full HTTP→SSE→AI SDK→processor pipeline.

- Export Provider.defaultLayer for test layer composition
- Add boot() helper for common service access (processor, session, provider)
- Extend TestLLMServer with usage support and httpError step type
- Tool abort test registers a real tool with hanging execute
- Reasoning test stays with in-process TestLLM (needs fine-grained events)