feat(routing): add weighted qwen3.5 overrides

[coniferconcepts]

Summary

• extend RoutingPolicy with a weighted strategy for weighted-router/<model> aliases while preserving the existing v1 single flow
• add strict weighted overrides for qwen3.5 so selection stays on the ollama/chutes allowlist, explicitly excludes go, and rewrites to a concrete provider before provider lock-in
• document the weighted override contract and add focused tests for validation, deterministic selection, passthrough compatibility, 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"
• local smoke test: 1000 weighted selections produced ollama=788, chutes=212, go=0

Guardrails

• weighted overrides require matching allowed_providers and weights
• excluded_providers are validated at load time and surfaced in route logging
• allow_global_fallback stays disabled for this v2 path, so unmatched or invalid weighted-router requests fail closed
• existing retry, cooldown, and credential rotation behavior remains unchanged after the model rewrite

Follow-Up

• if we want broader weighted routing later, we can add more model-specific overrides without changing the post-rewrite retry flow