fix: target directories retrieve

[mildred522]

Description

Fixes retrieval ignoring target_directories: when find/search passes target_uri/target_directories, the retriever now uses those directories as roots
and adds a URI prefix filter to prevent out-of-scope results. Adds a unit test to cover the expected semantics.

Type of Change

• 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 not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Honor target_directories in HierarchicalRetriever by using them as root_uris
• Add a URI prefix filter when target_directories is provided
• Add a unit test covering target_directories semantics

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published