feat(providers): add Eden AI provider

[MVS-source]

🎯 Description

Adds Eden AI (https://www.edenai.co) as a native provider. Eden AI is an OpenAI-compatible aggregator that exposes 100+ models from many providers (OpenAI, Anthropic, Google, Mistral, DeepSeek, X.AI, …) through a single EU-hosted endpoint and one API key.

EdenAIProvider mirrors the existing OpenAI-compatible providers (closest analog: OpenRouterProvider). Because Eden AI returns the actual per-request cost in each response, the provider uses that value for accurate accounting and falls back to a local pricing table when it's absent. Follows the "Adding a New Provider" checklist in CONTRIBUTING.

🔄 Type of Change

• 🚀 New provider integration
• ✅ Test additions/improvements
• 📝 Documentation update

🧪 Testing

Test cases added

• Unit tests (tests/test_edenai.py, 11 tests: init, completion, system prompt, API-cost vs local-pricing fallback, 401, 429, cost estimation)
• Manual testing (live end-to-end against the Eden AI API)

How to test

import asyncio
from cascadeflow.providers import EdenAIProvider  # uses EDENAI_API_KEY env var

async def main():
    p = EdenAIProvider()
    r = await p.complete(prompt="Hello!", model="anthropic/claude-sonnet-4-5")
    print(r.provider, r.model, r.cost, r.content)
    await p.close()

asyncio.run(main())

Verified locally: pytest tests/test_edenai.py → 11 passed; existing provider tests (groq/openai/ollama) → 49 passed, 1 skipped (no regressions); black/ruff clean. Live run: complete, stream, and fetch_available_models (763 models) all working; cost read from the API response.

📝 Notes

• Model naming uses Eden AI's provider/model format (e.g. mistral/codestral-latest).
• API key read from EDENAI_API_KEY only — never hardcoded or logged.
• Base URL defaults to https://api.edenai.run/v3 (EU-only endpoint available at https://api.eu.edenai.run/v3).