fix(retrieval): deduplicate hybrid results by document ID

[wang-cool]

Description

• Deduplicate overlapping text and vector retrieval results by document ID.
• Retain the vector-retrieved document and its similarity score when both retrieval paths return the same document.
• Add regression tests for overlapping and non-overlapping hybrid retrieval results.
• Fixes Hybrid retrieval may return duplicate documents from overlapping text and vector results #858

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.