feat: add EUrouter as AI provider

[DavidLoDico]

Summary

Adds EUrouter as an AI provider for BlockNote's AI features. EUrouter is an EU-based, GDPR-compliant AI gateway that provides access to multiple AI models through a single OpenAI-compatible API.

• Add eurouter.chat provider to the model playground server using createOpenAICompatible from @ai-sdk/openai-compatible (zero new dependencies)
• Add 7 EUrouter models to the playground dropdown (mistral-large-3, gpt-oss-120b, gpt-5-mini, claude-opus-4-6, green-r, deepseek-r1, kimi-k2.5)
• Add eurouter to the test AI model configuration
• Add EUROUTER_API_KEY to .env.example files

Changes

File	Change
packages/xl-ai-server/src/routes/model-playground/index.ts	Add eurouter.chat provider block using createOpenAICompatible
examples/09-ai/02-playground/src/data/aimodels.ts	Add 7 EUrouter models to the playground model list
packages/xl-ai/src/testUtil/testAIModels.ts	Add eurouter test model entry
packages/xl-ai-server/.env.example	Add EUROUTER_API_KEY
packages/xl-ai/.env.example	Add EUROUTER_API_KEY

Implementation Details

EUrouter exposes an OpenAI-compatible API at https://api.eurouter.ai/api/v1, so the integration follows the same pattern as the existing albert-etalab provider — using createOpenAICompatible() with no additional SDK dependencies. Authentication uses a Bearer token via the EUROUTER_API_KEY environment variable, which follows the existing getProviderKey() convention.

Test plan

• Verified streaming responses via curl against the EUrouter API
• Tested end-to-end in the AI playground (selected EUrouter model, triggered "Simplify" AI edit, confirmed diff view with Accept/Revert)
• CI passes

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