Fix datetime parsing for Unix timestamp strings in vectorstore_adapter

The parse_datetime function was failing when receiving Unix timestamps
as strings (e.g., '1765388915.12') because it tried to parse them as
ISO format strings. This fix tries to parse string values as floats
first before falling back to ISO format parsing.

Author: Andrew Brookins

agent_memory_server/vectorstore_adapter.py

@@ -403,6 +403,12 @@ def parse_datetime(dt_val: str | float | None) -> datetime | None:
                 # Unix timestamp from Redis
                 return datetime.fromtimestamp(dt_val, tz=UTC)
             if isinstance(dt_val, str):
+                # Try to parse as float first (Unix timestamp as string)
+                try:
+                    timestamp = float(dt_val)
+                    return datetime.fromtimestamp(timestamp, tz=UTC)
+                except ValueError:
+                    pass
                 # ISO string from other backends
                 return datetime.fromisoformat(dt_val)
             return None