test: cover reranker basics

[snoopuppy582]

Fixes #1697

Summary

• Added a dedicated tests/reranker package.
• Covered reranker base interfaces, factories, noop and cosine-local behavior.
• Added deterministic strategy tests for dialogue tracking, single-turn reconstruction, out-memory aggregation, background concatenation, and document-source concatenation.

Tests

• uv run --python 3.12 --extra mem-reader --with pytest pytest tests/reranker -q
• uv run --python 3.12 --extra mem-reader --with ruff ruff check tests/reranker
• git diff --check

Notes

• Kept the tests deterministic and local-only; HTTP reranker calls are not made.
• The contributing guide mentions a dev base branch, but this repository currently exposes main as the active/default base branch.

[CaralHsi]

Nice work! Thanks for your contribution!