feat: Add OpenAI provider support for embeddings and generation

Adds OpenAI provider to the unified provider architecture (ADR-015),
supporting:
- OpenAI API (api.openai.com)
- GitHub Models API (models.github.ai/inference)
- OpenAI-compatible endpoints (Fireworks, Together, etc.)

Features:
- Embedding support with text-embedding-3-small/large models
- Text generation via chat completions API
- Automatic retry with exponential backoff for rate limits
- Provider auto-detection in registry (priority after Bedrock)

Environment variables:
- OPENAI_API_KEY: API key (required)
- OPENAI_BASE_URL: Base URL override (optional)
- OPENAI_EMBEDDING_MODEL: Embedding model (default: text-embedding-3-small)
- OPENAI_GENERATION_MODEL: Generation model (default: gpt-4o-mini)

Also adds:
- Integration tests for RAG pipeline with MCP sampling
- MCP client sampling support for integration tests
- Ground truth Q&A pairs for Nextcloud User Manual

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/providers/__init__.py

@@ -4,12 +4,14 @@ from .anthropic import AnthropicProvider
 from .base import Provider
 from .bedrock import BedrockProvider
 from .ollama import OllamaProvider
+from .openai import OpenAIProvider
 from .registry import get_provider, reset_provider
 from .simple import SimpleProvider
 
 __all__ = [
     "Provider",
     "OllamaProvider",
+    "OpenAIProvider",
     "AnthropicProvider",
     "SimpleProvider",
     "BedrockProvider",