refactor(providers): address PR #772 review round 3 — hermetic test, lazy logging, defensive-guard tests

- test_registry.py: stub `mistralai.client.Mistral` in
  `test_registry_mistral_wins_over_ollama`, mirroring the sibling
  picker test, so the test doesn't depend on the SDK accepting
  arbitrary keys.
- openai.py: convert remaining f-string `logger.info(...)` calls to
  lazy `%s` formatting, aligning with the pattern in mistral.py and
  the repo's logging convention.
- test_mistral.py: add four tests covering the defensive RuntimeError
  guards in `embed()` and `_embed_batch_request()` — empty
  response.data, single null embedding, batch null embedding, and
  count-mismatch.
- docs/configuration.md: add `AWS_ACCESS_KEY_ID` and
  `AWS_SECRET_ACCESS_KEY` rows to the env-var reference table; they
  were already mentioned in prose but missing from the table.

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

tests/unit/providers/test_registry.py

@@ -108,8 +108,11 @@ def test_registry_openai_wins_over_mistral_and_ollama(clean_provider_env):
 
 
 @pytest.mark.unit
-def test_registry_mistral_wins_over_ollama(clean_provider_env):
+def test_registry_mistral_wins_over_ollama(clean_provider_env, mocker):
     """Mistral takes priority over Ollama when both are configured."""
+    # Stub the Mistral SDK constructor for the same reason as the sibling
+    # picker test — keeps the registry test independent of SDK key validation.
+    mocker.patch("nextcloud_mcp_server.providers.mistral.Mistral")
     clean_provider_env.setenv("MISTRAL_API_KEY", "mistral-key")
     clean_provider_env.setenv("OLLAMA_BASE_URL", "http://localhost:11434")
     _reload_config()