feat: Expose run_config of current invocation from ReadonlyContext

Merge #3410

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

- Closes: #3409

Co-authored-by: Hangfei Lin <hangfei@google.com>
COPYBARA_INTEGRATE_REVIEW=#3410 from ksachdeva:expose-runconfig b62b923
PiperOrigin-RevId: 828738445

Author: 2 people

src/google/adk/agents/readonly_context.py

@@ -24,6 +24,7 @@
 
   from ..sessions.session import Session
   from .invocation_context import InvocationContext
+  from .run_config import RunConfig
 
 
 class ReadonlyContext:
@@ -58,3 +59,8 @@ def state(self) -> MappingProxyType[str, Any]:
   def session(self) -> Session:
     """The current session for this invocation."""
     return self._invocation_context.session
+
+  @property
+  def run_config(self) -> Optional[RunConfig]:
+    """The run config of the current invocation. READONLY field."""
+    return self._invocation_context.run_config