Targeted router cleanup and regression coverage for contract path routing

[Copilot]

This PR performs a scoped cleanup in routing logic and adds regression coverage for a contract-routing edge case. The goal is to reduce dead logic in DocumentRouter while locking expected path behavior for contract-like credit-card documents.

• Routing logic cleanup

  • Removed unused locals in build_route_path() (contract_keywords, _text_lower read) that did not affect control flow or output.
  • Kept contract-path behavior intact: for Credit Card / Bank contract-like docs (amount absent), routing drops the {year} segment and resolves to issuer root.

• Regression test coverage

  • Added tests/test_router.py with a focused test asserting issuer-root routing for contract-like credit card input.
  • Test guards against future regressions that reintroduce year-segment routing for this case.

path = router.build_route_path(
    "Credit Card Statements",
    {"issuer": "AMEX (OZ)", "amount": "", "date": "2024-01-10"},
)

assert path == "Credit Card Statements\\AMEX (OZ)"