fix: add missing CheckpointRequest model for tests

- Added CheckpointRequest Pydantic model with session_id and phase fields
- Added phase validation against ALLOWED_PHASES
- Fixes test import error in test_summarize_and_phase.py

Author: AlmostBald-TRADING

src/contextforge_memory/main.py

@@ -347,6 +347,21 @@ def _select_embeddings_provider() -> EmbeddingsProvider:
 ALLOWED_PHASES = {"planning", "execution", "review"}
 
 
+class CheckpointRequest(BaseModel):
+    """Request model for checkpoint operations."""
+
+    session_id: str = Field(..., description="Session identifier")
+    phase: str = Field(..., description="Checkpoint phase")
+
+    @field_validator("phase")
+    @classmethod
+    def validate_phase(cls, v: str) -> str:
+        """Validate that phase is one of the allowed phases."""
+        if v not in ALLOWED_PHASES:
+            raise ValueError(f"Phase must be one of {sorted(ALLOWED_PHASES)}, got {v}")
+        return v
+
+
 def _get_env_int(key: str, default: int) -> int:
     """Get environment variable as integer with fallback to default.