feat(providers): add Mistral embedding provider, route registry through dynaconf

Adds a hosted Mistral embedding option (mistral-embed, 1024-dim) alongside
the existing Bedrock / OpenAI / Ollama / Simple providers. Implementation
mirrors OpenAIProvider: lazy dimension detection with a known-models lookup,
chunked batch requests, defensive index sort, and a 429-aware retry decorator.

In the same change, ProviderRegistry switches from os.getenv to the
dynaconf-backed Settings dataclass so all five providers share a single
configuration path. config.py gains the previously-uncovered Bedrock keys,
the new Mistral keys, the missing OPENAI_GENERATION_MODEL /
OLLAMA_GENERATION_MODEL, and SIMPLE_EMBEDDING_DIMENSION.

Auto-detection priority: Bedrock → OpenAI → Mistral → Ollama → Simple.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/providers/__init__.py

@@ -3,6 +3,7 @@
 from .anthropic import AnthropicProvider
 from .base import Provider
 from .bedrock import BedrockProvider
+from .mistral import MistralProvider
 from .ollama import OllamaProvider
 from .openai import OpenAIProvider
 from .registry import get_provider, reset_provider
@@ -13,6 +14,7 @@ __all__ = [
     "OllamaProvider",
     "OpenAIProvider",
     "AnthropicProvider",
+    "MistralProvider",
     "SimpleProvider",
     "BedrockProvider",
     "get_provider",