Add config compatibility matrix tests and document alias precedence

[Disaster-Terminator]

Motivation

• Ensure backward-compatible loading of legacy config keys while introducing clearer product terminology for entry_model/route_model and fallback_route_id/default_route.
• Prevent accidental regressions by codifying a small compatibility matrix and recursion guards in tests.
• Preserve existing runtime behavior unless tests detect an ambiguity or bug.

Description

• Added tests in tests/test_config.py covering entry_model only, route_model only, both keys present (deterministic precedence), fallback_route_id and legacy default_route alias, omitted target_model defaulting to the route_id, and missing fallback_route_id validation.
• Documented deterministic alias precedence in router/config.py comment indicating that route_model takes precedence via AliasChoices ordering for deterministic compatibility.
• Kept runtime logic unchanged other than a clarifying comment; test additions exercise recursive protections such as preventing route_model or target_model from referring back to configured route ids and requiring fallback_route_id to exist.

Testing

• Ran uv run python -m pytest -q and all tests passed (156 passed, 1 warning).
• Ran uv run python scripts/eval_routes.py --mock-embeddings and the evaluation harness passed all sample cases (10 eval case(s) passed).

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Codex Task

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