Expose multiple SearchType options via query param

[krapfj23]

Summary

Let API consumers choose which SearchType to use per request via a query parameter.

Blocked by

• Upgrade default search to GRAPH_COMPLETION #78 (upgrade default search to GRAPH_COMPLETION)

Blocks

Nothing

Steps

1. Add search_type query parameter to GET /documents/search with allowed values: insights, graph_completion, chunks, summaries
2. Default to graph_completion if not specified
3. Map the string param to cognee.SearchType enum
4. Update Pydantic response model to handle different result shapes (or use a union type)
5. Update API docs/examples for each search type
6. Write tests: each search type returns results in the expected format

Acceptance Criteria

• GET /documents/search?q=test&search_type=chunks returns raw chunks
• GET /documents/search?q=test&search_type=insights returns triplets
• Default (no param) uses graph_completion
• Tests cover each search type
• PR body includes Closes #79