Add config compatibility matrix tests for entry/route aliases

[Disaster-Terminator]

Motivation

• Ensure backward-compatible loading of new product-facing keys while preserving legacy keys and preventing regressions.
• Provide a compact compatibility matrix so future edits do not accidentally break existing local configs.

Description

• Added focused compatibility tests in tests/test_config.py covering entry_model and route_model alias handling and mixed-key behavior.
• Added tests for fallback_route_id and legacy default_route alias handling.
• Added a regression test verifying omitted target_model resolves to each route's route_id, and recursive-protection tests that entry_model/route_model cannot be a route_id and fallback_route_id must exist.
• Chose a deterministic compatibility rule that legacy route_model takes precedence when both route_model and entry_model are present, and documented that rule in the test code.

Testing

• Ran uv run python -m pytest -q, which passed: 157 passed, 1 warning.
• Ran uv run python scripts/eval_routes.py --mock-embeddings, which passed: 10 eval case(s) passed.

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Codex Task

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