feat: add EUrouter as AI provider

[DavidLoDico]

Summary

• Add EUrouter as an AI provider using the existing createOpenAICompatible pattern from @ai-sdk/openai-compatible
• EUrouter is an EU-based GDPR-compliant AI gateway that routes to multiple providers (OpenAI, Anthropic, Google, etc.) through a single OpenAI-compatible API
• No new dependencies required — reuses the existing @ai-sdk/openai-compatible package already used by albert-etalab

Changes

• Model playground route (packages/xl-ai-server/src/routes/model-playground/index.ts): Add eurouter.chat provider with base URL https://api.eurouter.ai/api/v1
• Playground model list (examples/09-ai/02-playground/src/data/aimodels.ts): Add representative EUrouter models (openai/gpt-4o, anthropic/claude-sonnet-4-5, google/gemini-2.5-pro)
• Environment config (packages/xl-ai-server/.env.example, packages/xl-ai/.env.example): Add EUROUTER_API_KEY
• Test models (packages/xl-ai/src/testUtil/testAIModels.ts): Add EUrouter test model entry

Test plan

• Set EUROUTER_API_KEY in .env and verify model playground can select and stream from EUrouter models
• Verify existing providers still work (no regressions)
• Existing tests pass

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