Preserve repeated facts query params

[omribz156]

Summary

• preserve repeated query parameters when forwarding facts requests to the control plane
• keep explicit forwarded query overrides replacing existing values
• add focused coverage for repeated params and override behavior

Fixes #299.

Verification

• .venv\Scripts\python -m pytest horizon\tests\test_facts_client.py
• .venv\Scripts\python -m pytest horizon\tests\test_facts_client.py horizon\tests\test_facts_router.py
• .venv\Scripts\python -m ruff check horizon\facts\client.py horizon\tests\test_facts_client.py
• git diff --check

Implemented with Codex assistance, with the patch kept focused and manually reviewed before opening.