fix: remove MaxLen constraint from observation content

The API Pydantic schema had a MaxLen(1000) constraint on observation
content while the database uses SQLAlchemy's Text type (unlimited).
This mismatch caused validation errors when observations exceeded
1000 characters (e.g., JSON schemas with 1458+ chars).

Removed the MaxLen constraint to match the DB schema. Retained:
- BeforeValidator(str.strip) for whitespace cleaning
- MinLen(1) to ensure non-empty content

Added comprehensive tests to verify:
- Long content (10K+ chars) is accepted
- Very long content (50K+ chars) is accepted
- Empty/whitespace-only content is still rejected
- Whitespace stripping still works

Fixes #385

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/schemas/base.py

@@ -183,7 +183,7 @@ ObservationStr = Annotated[
     str,
     BeforeValidator(str.strip),  # Clean whitespace
     MinLen(1),  # Ensure non-empty after stripping
-    MaxLen(1000),  # Keep reasonable length
+    # No MaxLen - matches DB Text column which has no length restriction
 ]