fix: reject negative recent-event limits

Merge https://github.com/google/adk-python/pull/6687

Unsupported negative limits should fail at configuration time, before a session backend handles them.

PiperOrigin-RevId: 968610704

src/google/adk/sessions/base_session_service.py

@@ -20,6 +20,7 @@ from typing import Optional
 
 from pydantic import BaseModel
 from pydantic import Field
+from pydantic import field_validator
 
 from ..events.event import Event
 from .session import Session
@@ -32,7 +33,8 @@ class GetSessionConfig(BaseModel):
   Attributes:
     num_recent_events: The limit of recent events to get for the session.
       Optional: if None, the filter is not applied; if greater than 0, returns
-        at most given number of recent events; if 0, no events are returned.
+        at most given number of recent events; if 0, no events are returned;
+        if negative, a ValueError is raised.
     after_timestamp: The earliest timestamp of events to get for the session.
       Optional: if None, the filter is not applied; otherwise, returns events
         with timestamp >= the given time.
@@ -41,6 +43,13 @@ class GetSessionConfig(BaseModel):
   num_recent_events: Optional[int] = None
   after_timestamp: Optional[float] = None
 
+  @field_validator('num_recent_events')
+  @classmethod
+  def _validate_num_recent_events(cls, value: Optional[int]) -> Optional[int]:
+    if value is not None and value < 0:
+      raise ValueError('num_recent_events must be greater than or equal to 0.')
+    return value
+
 
 class ListSessionsResponse(BaseModel):
   """The response of listing sessions.

tests/unittests/sessions/test_session_service.py

@@ -55,6 +55,12 @@ from . import _conformance
 from ._conformance import session_service  # noqa: F401
 
 
+def test_get_session_config_rejects_negative_num_recent_events():
+  """A negative recent-event limit is rejected at configuration time."""
+  with pytest.raises(ValueError, match='greater than or equal to 0'):
+    GetSessionConfig(num_recent_events=-1)
+
+
 class SessionServiceType(enum.Enum):
   IN_MEMORY = 'IN_MEMORY'
   DATABASE = 'DATABASE'