feat: add Salad AI Gateway provider

[mgorkii-nlplogix]

Description

Adds Salad AI Gateway as a native Continue provider backed by the OpenAI-compatible endpoint at https://ai.salad.cloud/v1/.

• Lists only qwen3.6-35b-a3b, with a 262,144-token context window
• Advertises image input, tool calling, reasoning, streaming, and parallel generation support
• Adds the provider to the core registry, OpenAI adapter, Add Model UI, VS Code schema, and provider docs
• Documents current per-token pricing: $0.09/M input tokens and $0.60/M output tokens
• Preserves the standard OpenAI-compatible request body instead of forcing reasoning off

Supersedes #12380.

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screen recording or screenshot

Not included because this adds provider/model metadata to the existing Add Model flow rather than a new interaction.

Tests

• CI=true node ./scripts/build-packages.js
• Core TypeScript and touched-file ESLint checks
• npm test -- --runInBand llm/toolSupport.test.ts (75 tests)
• npm run vitest -- llm/autodetect.vitest.ts (44 tests)
• npm run vitest -- llm/llms/OpenAI-compatible.vitest.ts (218 passed, 1 intentionally skipped for unsupported embeddings)
• OpenAI adapter build and focused tests (17 tests)
• GUI TypeScript, touched-file ESLint, and full test suite (372 tests)
• Prettier, JSON/schema alignment, SVG parse, and MDX parse checks

[mgorkii-nlplogix]

I have read the CLA Document and I hereby sign the CLA