feat: add SaladCloud AI Gateway provider

[mgorkii-nlplogix]

Ticket

N/A

Component/Service

• Web (Frontend)
• Jawn (Backend)
• Worker (Proxy)
• Bifrost (Marketing)
• AI Gateway
• Packages
• Infrastructure/Docker
• Documentation

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Performance improvement
• Refactoring

Deployment Notes

• No special deployment steps required
• Database migrations need to run
• Environment variable changes required
• Coordination with other teams needed

Extra Notes

This replaces and narrows #5674 to the currently advertised SaladCloud AI Gateway model only:

• model: qwen3.6-35b-a3b
• upstream endpoint: https://ai.salad.cloud/v1/chat/completions
• input: $0.09 / 1M tokens
• output: $0.60 / 1M tokens
• BYOK only; pass-through billing is disabled

The OpenAI-compatible request body is forwarded without forcing reasoning off, so caller-provided chat_template_kwargs remains intact.

Validation:

• cd packages && ../node_modules/.bin/jest __tests__/cost/usageProcessor.test.ts __tests__/cost/registrySnapshots.test.ts — 41 tests passed, 8 snapshots passed
• cd worker/test/ai-gateway && ../../../node_modules/.bin/vitest run registry-alibaba.spec.ts --config vitest.config.mts — 240 tests passed
• yarn tsc --noEmit -p worker/tsconfig.json — passed
• touched Worker and Web ESLint scopes — 0 errors
• git diff --check — passed

The repository-wide packages TypeScript check still reports three pre-existing errors in unrelated tests: a strict callback fixture in providers/helicone.test.ts, missing glob declarations, and a stale HeliconeRequest fixture.

Context

Adds first-class SaladCloud AI Gateway support to Helicone's provider registry, BYOK key UI, usage processing, domain mapping, model metadata, pricing, documentation, and gateway tests.

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