refactor(providers): address PR #772 review — shared retry, cleaner imports, no-op close

Addresses the Claude Code review on PR #772 plus the SonarCloud S1192 finding:

- Extract `retry_on_rate_limit` into `nextcloud_mcp_server/providers/_retry.py`
  as a parametric decorator. OpenAI and Mistral now share the same backoff
  loop; future providers can reuse it without copy-paste.
- New `tests/unit/providers/test_retry.py` covers the decorator: 429 retry +
  success, non-429 immediate re-raise, MAX_RETRIES exhaustion, default
  predicate, and unrelated exception passthrough.
- Tighten Mistral SDK import to `from mistralai.client.errors import SDKError`
  (the canonical sub-path; the reviewer's `from mistralai.models import
  SDKError` does not exist in mistralai 2.4.5).
- Replace `MistralProvider.close()`'s direct `__aexit__` call with a no-op +
  comment — the Speakeasy-generated client has no public close hook and the
  underlying httpx client is closed by GC.
- Extract the duplicated "Embedding not supported" message to a module-level
  constant (SonarCloud S1192).
- Align `Settings.get_embedding_model_name()` Bedrock check with the registry
  by also considering `bedrock_generation_model`.
- Add the `mock_mistral_client` fixture to
  `test_mistral_no_embeddings_disabled` for parity with the rest of the file.

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

nextcloud_mcp_server/config.py

@@ -613,7 +613,11 @@ class Settings:
         Returns:
             Active embedding model name
         """
-        if self.aws_region or self.bedrock_embedding_model:
+        if (
+            self.aws_region
+            or self.bedrock_embedding_model
+            or self.bedrock_generation_model
+        ):
             return self.bedrock_embedding_model or "bedrock-default"
 
         if self.openai_api_key: