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/registry.py

@@ -6,6 +6,7 @@ import os
 from .base import Provider
 from .bedrock import BedrockProvider
 from .ollama import OllamaProvider
+from .openai import OpenAIProvider
 from .simple import SimpleProvider
 
 logger = logging.getLogger(__name__)
@@ -17,8 +18,9 @@ class ProviderRegistry:
 
     Checks environment variables in priority order and creates appropriate provider:
     1. Bedrock (AWS_REGION + BEDROCK_*_MODEL)
-    2. Ollama (OLLAMA_BASE_URL)
-    3. Simple (fallback for testing/development)
+    2. OpenAI (OPENAI_API_KEY)
+    3. Ollama (OLLAMA_BASE_URL)
+    4. Simple (fallback for testing/development)
     """
 
     @staticmethod
@@ -28,8 +30,9 @@ class ProviderRegistry:
 
         Priority order:
         1. Bedrock - if AWS_REGION or BEDROCK_EMBEDDING_MODEL is set
-        2. Ollama - if OLLAMA_BASE_URL is set
-        3. Simple - fallback for testing/development
+        2. OpenAI - if OPENAI_API_KEY is set
+        3. Ollama - if OLLAMA_BASE_URL is set
+        4. Simple - fallback for testing/development
 
         Returns:
             Provider instance
@@ -42,6 +45,12 @@ class ProviderRegistry:
                 - BEDROCK_EMBEDDING_MODEL: Model ID for embeddings (e.g., "amazon.titan-embed-text-v2:0")
                 - BEDROCK_GENERATION_MODEL: Model ID for text generation (e.g., "anthropic.claude-3-sonnet-20240229-v1:0")
 
+            OpenAI:
+                - OPENAI_API_KEY: OpenAI API key (or GITHUB_TOKEN for GitHub Models)
+                - OPENAI_BASE_URL: Base URL override (e.g., "https://models.github.ai/inference")
+                - OPENAI_EMBEDDING_MODEL: Model for embeddings (default: "text-embedding-3-small")
+                - OPENAI_GENERATION_MODEL: Model for text generation (e.g., "gpt-4o-mini")
+
             Ollama:
                 - OLLAMA_BASE_URL: Ollama API base URL (e.g., "http://localhost:11434")
                 - OLLAMA_EMBEDDING_MODEL: Model for embeddings (default: "nomic-embed-text")
@@ -70,7 +79,28 @@ class ProviderRegistry:
                 aws_secret_access_key=os.getenv("AWS_SECRET_ACCESS_KEY"),
             )
 
-        # 2. Check for Ollama
+        # 2. Check for OpenAI
+        openai_api_key = os.getenv("OPENAI_API_KEY")
+        if openai_api_key:
+            base_url = os.getenv("OPENAI_BASE_URL")
+            embedding_model = os.getenv(
+                "OPENAI_EMBEDDING_MODEL", "text-embedding-3-small"
+            )
+            generation_model = os.getenv("OPENAI_GENERATION_MODEL")
+
+            logger.info(
+                f"Using OpenAI provider: base_url={base_url or 'default'}, "
+                f"embedding_model={embedding_model}, "
+                f"generation_model={generation_model}"
+            )
+            return OpenAIProvider(
+                api_key=openai_api_key,
+                base_url=base_url,
+                embedding_model=embedding_model,
+                generation_model=generation_model,
+            )
+
+        # 3. Check for Ollama (local LLM)
         ollama_url = os.getenv("OLLAMA_BASE_URL")
         if ollama_url:
             embedding_model = os.getenv("OLLAMA_EMBEDDING_MODEL", "nomic-embed-text")
@@ -89,12 +119,12 @@ class ProviderRegistry:
                 verify_ssl=verify_ssl,
             )
 
-        # 3. Fallback to Simple provider for development/testing
+        # 4. Fallback to Simple provider for development/testing
         dimension = int(os.getenv("SIMPLE_EMBEDDING_DIMENSION", "384"))
         logger.warning(
-            "No provider configured (AWS_REGION, OLLAMA_BASE_URL not set). "
+            "No provider configured (AWS_REGION, OPENAI_API_KEY, OLLAMA_BASE_URL not set). "
             "Using SimpleProvider for testing/development. "
-            "For production, configure Bedrock or Ollama."
+            "For production, configure Bedrock, OpenAI, or Ollama."
         )
         return SimpleProvider(dimension=dimension)