docs(providers): document CoreWeave custom endpoint setup

[crwvrosenblum]

What does this PR do?

Add a CoreWeave Serverless Inference recipe to the existing custom-provider cookbook. It uses providers.coreweave / custom:coreweave, the W&B inference endpoint and WANDB_API_KEY, with optional endpoint-scoped project attribution.

The guide distinguishes the models.dev metadata entry from a registered runtime provider. It recommends v2026.9.21 or later for auxiliary headers, credits the shipped #113969 fix, and explains using a separate named provider when changing endpoints. Sending a project header is separate from checking billing/usage attribution.

Related Issue

#43964 and #44250. This documents the supported custom-provider route and adds no vendor plugin. The remaining auxiliary endpoint-override correction is tracked separately in #113397.

Changes Made

Only website/docs/integrations/providers.md changes. The existing cookbook anchor is preserved.

Validation

• Both YAML variants parse. Real config/runtime resolution, main-client construction, model discovery and auxiliary resolution passed offline on v2026.9.21 and current main. Bare provider: wandb is rejected as expected.
• Bounded live checks on unmodified v2026.9.21 passed authentication, 30-model discovery, a native read_file dispatch and tool-result roundtrip, synchronous auxiliary streaming, asynchronous auxiliary chat and the expected unavailable-model 404. Every request carried the configured project header to the inference endpoint.
• This verifies the provider/client route and real tool dispatch. Interactive CLI behavior, a full autonomous coding task, streamed tool arguments, other models and billing-UI attribution were not tested.
• The prior 83 offline runtime/header checks remain recorded. The full website build and full repository suite were not run.

Checklist

• Existing custom-provider configuration used.
• Released-header caveat updated from verified behavior.
• Cookbook anchor and one-file scope preserved.
• Full website build and full repository suite.

[crwvrosenblum]

@teknium1