fix: restore API backward compatibility for v0.18.x clients (#638)

v0.18.5 clients (homebrew) fail against v0.19.0 servers because:
- `entity_type` was renamed to `note_type` in EntityResponse
- 13 fields gained `Field(exclude=True)` and vanished from JSON

🔧 EntityResponse: add `entity_type` computed_field mirroring `note_type`
🔧 memory.py: remove `exclude=True` from 13 fields across EntitySummary,
   RelationSummary, ObservationSummary, and MemoryMetadata
🔧 Add v0.18 backward-compat contract tests

Closes #638

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez

src/basic_memory/schemas/memory.py

@@ -125,7 +125,8 @@ class EntitySummary(BaseModel):
 
     type: Literal["entity"] = "entity"
     external_id: str  # UUID for v2 API routing
-    entity_id: Optional[int] = Field(None, exclude=True)  # Internal DB ID
+    # COMPAT(v0.18): old clients expect these fields in JSON
+    entity_id: Optional[int] = None
     permalink: Optional[str]
     title: str
     content: Optional[str] = None
@@ -143,18 +144,19 @@ class RelationSummary(BaseModel):
     """Simplified relation representation."""
 
     type: Literal["relation"] = "relation"
-    relation_id: Optional[int] = Field(None, exclude=True)  # Internal DB ID
-    entity_id: Optional[int] = Field(None, exclude=True)  # Internal FK
+    # COMPAT(v0.18): old clients expect these fields in JSON
+    relation_id: Optional[int] = None
+    entity_id: Optional[int] = None
     title: str
     file_path: str
     permalink: str
     relation_type: str
     from_entity: Optional[str] = None
-    from_entity_id: Optional[int] = Field(None, exclude=True)  # Internal FK
-    from_entity_external_id: Optional[str] = Field(None, exclude=True)  # Internal routing ID
+    from_entity_id: Optional[int] = None
+    from_entity_external_id: Optional[str] = None
     to_entity: Optional[str] = None
-    to_entity_id: Optional[int] = Field(None, exclude=True)  # Internal FK
-    to_entity_external_id: Optional[str] = Field(None, exclude=True)  # Internal routing ID
+    to_entity_id: Optional[int] = None
+    to_entity_external_id: Optional[str] = None
     created_at: Annotated[
         datetime, Field(json_schema_extra={"type": "string", "format": "date-time"})
     ]
@@ -168,10 +170,11 @@ class ObservationSummary(BaseModel):
     """Simplified observation representation."""
 
     type: Literal["observation"] = "observation"
-    observation_id: Optional[int] = Field(None, exclude=True)  # Internal DB ID
-    entity_id: Optional[int] = Field(None, exclude=True)  # Internal FK
-    entity_external_id: Optional[str] = Field(None, exclude=True)  # Internal routing ID
-    title: Optional[str] = Field(None, exclude=True)  # Redundant with parent entity
+    # COMPAT(v0.18): old clients expect these fields in JSON
+    observation_id: Optional[int] = None
+    entity_id: Optional[int] = None
+    entity_external_id: Optional[str] = None
+    title: Optional[str] = None
     file_path: str
     permalink: str
     category: str
@@ -192,13 +195,18 @@ class MemoryMetadata(BaseModel):
     types: Optional[List[SearchItemType]] = None
     depth: int
     timeframe: Optional[str] = None
-    generated_at: Optional[datetime] = Field(None, exclude=True)  # Internal timing
-    primary_count: Optional[int] = None  # Changed field name
-    related_count: Optional[int] = None  # Changed field name
-    total_results: Optional[int] = Field(None, exclude=True)  # Internal counter
+    # COMPAT(v0.18): old clients expect generated_at and total_results in JSON
+    generated_at: Optional[datetime] = None
+    primary_count: Optional[int] = None
+    related_count: Optional[int] = None
+    total_results: Optional[int] = None
     total_relations: Optional[int] = None
     total_observations: Optional[int] = None
 
+    @field_serializer("generated_at")
+    def serialize_generated_at(self, dt: Optional[datetime]) -> Optional[str]:
+        return dt.isoformat() if dt else None
+
 
 class ContextResult(BaseModel):
     """Context result containing a primary item with its observations and related items."""

src/basic_memory/schemas/response.py

@@ -14,7 +14,7 @@
 from datetime import datetime
 from typing import List, Optional, Dict
 
-from pydantic import BaseModel, ConfigDict, Field, model_validator
+from pydantic import BaseModel, ConfigDict, Field, computed_field, model_validator
 
 from basic_memory.schemas.base import Relation, Permalink, NoteType, ContentType, Observation
 
@@ -192,6 +192,13 @@ class EntityResponse(SQLAlchemyModel):
     title: str
     file_path: str
     note_type: NoteType
+
+    # COMPAT(v0.18): old clients expect entity_type; remove when no longer needed
+    @computed_field  # type: ignore[prop-decorator]
+    @property
+    def entity_type(self) -> str:
+        return self.note_type
+
     entity_metadata: Optional[Dict] = None
     checksum: Optional[str] = None
     content_type: ContentType