Make Runner an abstract base class

src/agents/__init__.py

@@ -47,7 +47,7 @@
 from .models.openai_responses import OpenAIResponsesModel
 from .repl import run_demo_loop
 from .result import RunResult, RunResultStreaming
-from .run import RunConfig, Runner
+from .run import DefaultRunner, RunConfig, Runner, set_default_runner
 from .run_context import RunContextWrapper, TContext
 from .stream_events import (
     AgentUpdatedStreamEvent,
@@ -103,6 +103,7 @@
     handoff_span,
     mcp_tools_span,
     set_trace_processors,
+    set_trace_provider,
     set_tracing_disabled,
     set_tracing_export_api_key,
     speech_group_span,
@@ -161,6 +162,7 @@ def enable_verbose_stdout_logging():
     "ToolsToFinalOutputFunction",
     "ToolsToFinalOutputResult",
     "Runner",
+    "DefaultRunner",
     "run_demo_loop",
     "Model",
     "ModelProvider",
@@ -239,9 +241,11 @@ def enable_verbose_stdout_logging():
     "generation_span",
     "get_current_span",
     "get_current_trace",
+    "get_default_runner",
     "guardrail_span",
     "handoff_span",
     "set_trace_processors",
+    "set_trace_provider",
     "set_tracing_disabled",
     "speech_group_span",
     "transcription_span",
@@ -266,6 +270,7 @@ def enable_verbose_stdout_logging():
     "set_default_openai_key",
     "set_default_openai_client",
     "set_default_openai_api",
+    "set_default_runner",
     "set_tracing_export_api_key",
     "enable_verbose_stdout_logging",
     "gen_trace_id",