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>

tests/rag_evaluation/llm_providers.py

@@ -3,8 +3,8 @@
 DEPRECATED: This module is maintained for backward compatibility with RAG evaluation tests.
 New code should use nextcloud_mcp_server.providers directly.
 
-Supports Ollama (local), Anthropic (cloud), and Bedrock (AWS) providers for both ground truth
-generation and evaluation.
+Supports Ollama (local), Anthropic (cloud), Bedrock (AWS), and OpenAI (cloud) providers
+for both ground truth generation and evaluation.
 """
 
 import os
@@ -13,6 +13,7 @@ from nextcloud_mcp_server.providers import (
     AnthropicProvider,
     BedrockProvider,
     OllamaProvider,
+    OpenAIProvider,
     Provider,
 )
 
@@ -25,11 +26,14 @@ def create_llm_provider(
     anthropic_model: str | None = None,
     bedrock_region: str | None = None,
     bedrock_model: str | None = None,
+    openai_api_key: str | None = None,
+    openai_base_url: str | None = None,
+    openai_model: str | None = None,
 ) -> Provider:
     """Create an LLM provider from environment variables or arguments.
 
     Args:
-        provider: Provider type ('ollama', 'anthropic', or 'bedrock').
+        provider: Provider type ('ollama', 'anthropic', 'bedrock', or 'openai').
             Defaults to RAG_EVAL_PROVIDER env var or 'ollama'
         ollama_base_url: Ollama base URL. Defaults to RAG_EVAL_OLLAMA_BASE_URL or 'http://localhost:11434'
         ollama_model: Ollama model. Defaults to RAG_EVAL_OLLAMA_MODEL or 'llama3.2:1b'
@@ -38,6 +42,9 @@ def create_llm_provider(
         bedrock_region: AWS region. Defaults to RAG_EVAL_BEDROCK_REGION or AWS_REGION env var
         bedrock_model: Bedrock model ID. Defaults to RAG_EVAL_BEDROCK_MODEL or
             'anthropic.claude-3-sonnet-20240229-v1:0'
+        openai_api_key: OpenAI API key. Defaults to OPENAI_API_KEY env var
+        openai_base_url: OpenAI base URL. Defaults to OPENAI_BASE_URL (for GitHub Models API)
+        openai_model: OpenAI model. Defaults to OPENAI_GENERATION_MODEL or 'gpt-4o-mini'
 
     Returns:
         Provider instance
@@ -83,7 +90,22 @@ def create_llm_provider(
             region_name=region, embedding_model=None, generation_model=model
         )
 
+    elif provider == "openai":
+        api_key = openai_api_key or os.environ.get("OPENAI_API_KEY")
+        if not api_key:
+            raise ValueError(
+                "OpenAI API key required. Set OPENAI_API_KEY environment variable."
+            )
+        base_url = openai_base_url or os.environ.get("OPENAI_BASE_URL")
+        model = openai_model or os.environ.get("OPENAI_GENERATION_MODEL", "gpt-4o-mini")
+        return OpenAIProvider(
+            api_key=api_key,
+            base_url=base_url,
+            embedding_model=None,
+            generation_model=model,
+        )
+
     else:
         raise ValueError(
-            f"Invalid provider: {provider}. Must be 'ollama', 'anthropic', or 'bedrock'."
+            f"Invalid provider: {provider}. Must be 'ollama', 'anthropic', 'bedrock', or 'openai'."
         )