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>

uv.lock

@@ -1951,6 +1951,7 @@ dependencies = [
     { name = "jinja2" },
     { name = "langchain-text-splitters" },
     { name = "mcp", extra = ["cli"] },
+    { name = "openai" },
     { name = "opentelemetry-api" },
     { name = "opentelemetry-exporter-otlp-proto-grpc" },
     { name = "opentelemetry-instrumentation-asgi" },
@@ -1999,6 +2000,7 @@ requires-dist = [
     { name = "jinja2", specifier = ">=3.1.6" },
     { name = "langchain-text-splitters", specifier = ">=1.0.0" },
     { name = "mcp", extras = ["cli"], specifier = ">=1.22,<1.23" },
+    { name = "openai", specifier = ">=2.8.1" },
     { name = "opentelemetry-api", specifier = ">=1.28.2" },
     { name = "opentelemetry-exporter-otlp-proto-grpc", specifier = ">=1.28.2" },
     { name = "opentelemetry-instrumentation-asgi", specifier = ">=0.49b2" },
@@ -2146,6 +2148,25 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/b6/ca/862b1e7a639460f0ca25fd5b6135fb42cf9deea86d398a92e44dfda2279d/onnxruntime-1.23.2-cp313-cp313t-manylinux_2_27_x86_64.manylinux_2_28_x86_64.whl", hash = "sha256:e2b9233c4947907fd1818d0e581c049c41ccc39b2856cc942ff6d26317cee145", size = 17394184, upload-time = "2025-10-22T03:47:08.127Z" },
 ]
 
+[[package]]
+name = "openai"
+version = "2.8.1"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "anyio" },
+    { name = "distro" },
+    { name = "httpx" },
+    { name = "jiter" },
+    { name = "pydantic" },
+    { name = "sniffio" },
+    { name = "tqdm" },
+    { name = "typing-extensions" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/d5/e4/42591e356f1d53c568418dc7e30dcda7be31dd5a4d570bca22acb0525862/openai-2.8.1.tar.gz", hash = "sha256:cb1b79eef6e809f6da326a7ef6038719e35aa944c42d081807bfa1be8060f15f", size = 602490, upload-time = "2025-11-17T22:39:59.549Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/55/4f/dbc0c124c40cb390508a82770fb9f6e3ed162560181a85089191a851c59a/openai-2.8.1-py3-none-any.whl", hash = "sha256:c6c3b5a04994734386e8dad3c00a393f56d3b68a27cd2e8acae91a59e4122463", size = 1022688, upload-time = "2025-11-17T22:39:57.675Z" },
+]
+
 [[package]]
 name = "opentelemetry-api"
 version = "1.38.0"