- Introduce a DEFAULT_RUNNER global and a set_default_runner() function to allow callers to specify a default Runner for agent runs.
- Update Runner.run, run_sync, and run_streaming to use DEFAULT_RUNNER if set, otherwise fallback to DefaultRunner.
- Add detailed docstrings to run, run_sync, and run_streaming methods for clarity on agent execution flow and exceptions.

This commit renames the abstract and concrete methods run_impl, run_sync_impl, and run_streamed_impl in the Runner and DefaultRunner classes to _run_impl, _run_sync_impl, and _run_streamed_impl (or _start_streaming as appropriate) for improved naming consistency. Updates all method calls and test mocks accordingly. No behavior changes.

…lace global with accessor

- Move TraceProvider and related logic from setup.py to new provider.py module
- Replace direct GLOBAL_TRACE_PROVIDER references with get_trace_provider() accessor throughout tracing code
- Update init and util imports to use get_trace_provider
- Call set_trace_provider(TraceProvider()) on init
- Remove ID and time generation logic from util, delegate to TraceProvider
- Update SpanImpl to always use passed-in span_id

…tructor