feat (provider/neon): add Neon AI Gateway provider

[andrelandgraf]

Summary

Adds @ai-sdk/neon, a provider for the branch-scoped Neon AI Gateway.

Each Neon project branch gets its own gateway host, and a platform token authorizes requests for that branch. The provider routes each model to the best gateway endpoint based on its id, so a single neon('databricks-...') call (same base URL + token) reaches the full databricks-* catalog with the best available feature support.

Routing

Model family	Endpoint	Why
Anthropic (databricks-claude-*)	native Messages API	streaming structured output + native reasoning
OpenAI (databricks-gpt-*, *-codex)	native Responses API	Codex (only served natively), native reasoning, image-gen tool
Everything else (Gemini, Llama, Qwen, gpt-oss, ...)	unified MLflow (OpenAI-compatible) endpoint	broad coverage; Gemini is here because its native gateway endpoint can't stream

Routing reuses the official @ai-sdk/anthropic and @ai-sdk/openai model classes (no reimplementation), with thin Neon subclasses that handle gateway quirks:

• OpenAI: sets forceReasoning for gpt-5* so reasoning handling works despite the required databricks- prefix.
• Anthropic: disables eager_input_streaming (rejected by the gateway) so streaming tool calls work.

Configuration

NEON_AI_GATEWAY_BASE_URL (branch host root) + NEON_AI_GATEWAY_TOKEN (platform token), both from the Console's AI Gateway tab; or pass baseURL/apiKey to createNeon.

import { neon } from '@ai-sdk/neon';
import { generateText } from 'ai';

const { text } = await generateText({
  model: neon('databricks-gpt-5-3-codex'), // or claude / gemini / llama / ...
  prompt: 'Summarize Postgres for me.',
});

Verified live (Anthropic, OpenAI, Codex, Google, Meta)

generateText, streamText, system prompts, multi-turn, tool calling (single + multi-step + streaming), generateObject, streamObject, reasoning, and image (vision) input.

For MLflow-routed models, the provider detects the family and drops parameters a backend rejects (e.g. penalties/seed for Llama, reasoningEffort for Gemini) with an AI SDK warning instead of a hard error.

Known gateway limitations (not adapter bugs; documented)

• Image generation (generateImage) and embeddings (embed/embedMany) have no gateway endpoint → NoSuchModelError. (Image gen exists only as the OpenAI Responses image_generation tool and is constrained by the gateway's response-size cap.)
• gpt-oss-* returns a non-standard response shape on the unified endpoint.
• databricks-gpt-5-mini + multi-step tools intermittently returns gateway 502s (upstream); Codex and other models are unaffected.

Test plan

• pnpm --filter @ai-sdk/neon test (node + edge), type-check, build
• oxlint + oxfmt clean
• Live end-to-end capability matrix across Anthropic, OpenAI, Codex, Google, and Meta models
• Verified as an external consumer (linked package) in a separate project

Note: per contributing/add-new-provider.md, first-party @ai-sdk/<provider> packages may warrant an issue to discuss first — happy to open one / adjust as maintainers prefer.