chore: sets default to 1 for last_k in search (#1412)

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## Description
Adds default value for last_k in search to avoid unwanted failure when
users don't provide the needed parameter.

## Type of Change
<!-- Please check the relevant option -->
- [ ] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [x] Code refactoring
- [ ] Performance improvement
- [ ] Other (please specify):

## Changes Made
Add default value to last_k.

## Testing
Manual + tests are already present for the funtionality

## Screenshots/Videos (if applicable)
None

## Pre-submission Checklist
<!-- Please check all boxes that apply before submitting your PR -->
- [x] **I have tested my changes thoroughly before submitting this PR**
- [x] **This PR contains minimal changes necessary to address the
issue/feature**
- [x] My code follows the project's coding standards and style
guidelines
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] I have added necessary documentation (if applicable)
- [x] All new and existing tests pass
- [x] I have searched existing PRs to ensure this change hasn't been
submitted already
- [x] I have linked any relevant issues in the description
- [x] My commits have clear and descriptive messages

## Related Issues
None

## Additional Notes
None

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin.

cognee/api/v1/search/search.py

@@ -22,7 +22,7 @@ async def search(
     node_type: Optional[Type] = NodeSet,
     node_name: Optional[List[str]] = None,
     save_interaction: bool = False,
-    last_k: Optional[int] = None,
+    last_k: Optional[int] = 1,
     only_context: bool = False,
     use_combined_context: bool = False,
 ) -> Union[List[SearchResult], CombinedSearchResult]: