fix: Add ISO datetime serialization to MCP schema models

Fixes MCP client schema validation failures for build_context tool
by ensuring datetime fields are serialized as ISO format strings.

MCP clients expect "date-time" format validation, which requires
datetime objects to be serialized as ISO strings (e.g. "2023-12-08T10:30:00")
rather than Python datetime objects.

Changes:
- Added ConfigDict with json_encoders to EntitySummary, RelationSummary,
  ObservationSummary, and MemoryMetadata models
- Ensures datetime fields serialize as dt.isoformat() strings

Resolves persistent schema validation issues reported in #263

Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: jope-bm <jope-bm@users.noreply.github.com>

Author: claude[bot]

src/basic_memory/schemas/memory.py

@@ -4,7 +4,7 @@
 from typing import List, Optional, Annotated, Sequence, Literal, Union
 
 from annotated_types import MinLen, MaxLen
-from pydantic import BaseModel, Field, BeforeValidator, TypeAdapter
+from pydantic import BaseModel, Field, BeforeValidator, TypeAdapter, ConfigDict
 
 from basic_memory.schemas.search import SearchItemType
 
@@ -117,6 +117,8 @@ def memory_url_path(url: memory_url) -> str:  # pyright: ignore
 
 class EntitySummary(BaseModel):
     """Simplified entity representation."""
+    
+    model_config = ConfigDict(json_encoders={datetime: lambda dt: dt.isoformat()})
 
     type: Literal["entity"] = "entity"
     permalink: Optional[str]
@@ -128,6 +130,8 @@ class EntitySummary(BaseModel):
 
 class RelationSummary(BaseModel):
     """Simplified relation representation."""
+    
+    model_config = ConfigDict(json_encoders={datetime: lambda dt: dt.isoformat()})
 
     type: Literal["relation"] = "relation"
     title: str
@@ -141,6 +145,8 @@ class RelationSummary(BaseModel):
 
 class ObservationSummary(BaseModel):
     """Simplified observation representation."""
+    
+    model_config = ConfigDict(json_encoders={datetime: lambda dt: dt.isoformat()})
 
     type: Literal["observation"] = "observation"
     title: str
@@ -153,6 +159,8 @@ class ObservationSummary(BaseModel):
 
 class MemoryMetadata(BaseModel):
     """Simplified response metadata."""
+    
+    model_config = ConfigDict(json_encoders={datetime: lambda dt: dt.isoformat()})
 
     uri: Optional[str] = None
     types: Optional[List[SearchItemType]] = None