fix(embedding): normalize gateway base_url to the /v1 base path

EMBEDDING_GATEWAY_URL is configured as a bare origin (scheme://host:port) —
the deployment's Service URL. GatewayProvider now appends the gateway's /v1
base path before handing the URL to the OpenAI SDK, so both embed posts
({base}/embeddings) and dimension discovery ({base}/models) land under /v1.
Idempotent: a URL already ending in /v1 is left unchanged.

This lets EMBEDDING_GATEWAY_URL stay a bare domain (matching the gitops
Service URLs) instead of requiring a hand-appended /v1.

Also align the `embedding_gateway_model` field default with _DEFAULTS
("mistral/mistral-embed"). The gateway catalog is provider-namespaced, and
_detect_dimension matches `entry.id == embedding_model`; the stale
un-namespaced default would silently miss the catalog entry and leave the
dimension unresolved (re-triggering the external-mode startup crash).

Tests: bare / trailing-slash / idempotent normalization + a bare-origin
discovery test asserting /v1/models. 16 gateway-provider tests pass;
providers + vector suites green (129 total); ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/config.py

@@ -696,7 +696,9 @@ class Settings:
     fact_event_emitter: str = "none"  # none | nats | stdout
     ingest_bus_url: str | None = None  # required when ingest_mode=external
     embedding_gateway_url: str | None = None  # required when provider=gateway
-    embedding_gateway_model: str = "mistral-embed"  # logical model gateway routes
+    embedding_gateway_model: str = (
+        "mistral/mistral-embed"  # provider-namespaced id the gateway routes on
+    )
     # Gateway M2M OIDC client creds (separate realm; see _DEFAULTS comment).
     embedding_gateway_token_url: str | None = None
     embedding_gateway_client_id: str | None = None