* fix(main): stop per-request custom pricing from clobbering shared model_cost pricing
A request routed through a wildcard deployment with explicit zero pricing
(e.g. openai/* with input_cost_per_token: 0) registered that pricing on the
shared {provider}/{model} key in litellm.model_cost, so sibling deployments
relying on built-in pricing logged $0 until process restart (LIT-3991).
Request-time registration in completion()/embedding() now mirrors the
router-startup isolation: router-originated requests register full pricing
under the deployment's unique model id only, while the shared backend key
receives the entry with custom pricing fields stripped. Direct SDK calls
without a router deployment id keep the legacy shared-key registration.
The stripping logic is shared via
CustomPricingLiteLLMParams.strip_custom_pricing_fields and reused by
Router._create_deployment and Router.add_deployment.
* test: update legacy tests that asserted per-request pricing leaking into shared model_cost
test_router_fallbacks_with_custom_model_costs asserted the shared
claude-sonnet-4-5-20250929 entry ends up with the deployment's 30/60
pricing, which is exactly the cross-deployment leak this PR removes; it
now asserts the shared key keeps the built-in pricing, matching the
test's stated goal.
test_cost_calc.py::test_run computed streaming cost via
completion_cost(response), which only matched the non-stream cost while
the shared gpt-3.5-turbo entry was poisoned with the per-request
2/token pricing; it now passes the request's custom pricing explicitly
via custom_cost_per_token.
