feat(routing): add single-provider weighted-router overrides

[coniferconcepts]

Summary

• add a v1 RoutingPolicy that rewrites weighted-router/<model> into a concrete provider-prefixed model before provider lock-in
• support strict single overrides via MODEL_ROUTING_OVERRIDES, including load-time validation and fail-closed behavior for unmatched weighted-router/* models
• preserve the existing retry, cooldown, and credential rotation flow once the model has been rewritten
• document the override contract and add focused tests for validation, env loading, passthrough behavior, and dispatch-time rewriting

Testing

• pytest tests/test_routing_policy.py tests/test_client_routing_policy.py
• python3 -m py_compile "src/rotator_library/routing_policy.py" "src/rotator_library/client.py" "tests/test_routing_policy.py" "tests/test_client_routing_policy.py"

V1 Scope

• only single strategy is supported
• allowed_providers must contain only the primary provider
• fallback_providers must remain empty
• unmatched weighted-router/* models fail closed instead of silently falling through

Follow-Up

• add weighted override support for models like qwen3.5
• consider moving from env-only overrides to a first-class config surface if this pattern sticks