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>

tests/unit/providers/test_gateway_provider.py

@@ -289,3 +289,59 @@ async def test_detect_dimension_skips_when_already_known(monkeypatch):
     await provider._detect_dimension()
     assert called["n"] == 0
     assert provider.get_dimension() == 1024
+
+
+# --- /v1 base-path normalization --------------------------------------------
+# EMBEDDING_GATEWAY_URL is configured as a bare origin (scheme://host:port);
+# the provider appends the gateway's /v1 base path so both the OpenAI SDK's
+# embed posts ({base}/embeddings) and discovery ({base}/models) land under /v1.
+
+
+def _client_base(provider: GatewayProvider) -> str:
+    return str(provider.client.base_url).rstrip("/")
+
+
+def test_bare_base_url_gets_v1_base_path():
+    provider = GatewayProvider(
+        base_url="http://gw:8083", embedding_model="mistral/mistral-embed"
+    )
+    assert _client_base(provider).endswith("/v1")
+
+
+def test_v1_base_url_is_idempotent():
+    # A URL that already carries /v1 (e.g. legacy config) is not doubled.
+    provider = GatewayProvider(
+        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+    )
+    base = _client_base(provider)
+    assert base.endswith("/v1")
+    assert not base.endswith("/v1/v1")
+
+
+def test_trailing_slash_base_url_normalized():
+    provider = GatewayProvider(
+        base_url="http://gw:8083/", embedding_model="mistral/mistral-embed"
+    )
+    base = _client_base(provider)
+    assert base.endswith("/v1")
+    assert not base.endswith("/v1/v1")
+
+
+async def test_detect_dimension_with_bare_base_url_hits_v1_models(monkeypatch):
+    """End-to-end of the fix: a bare-origin base_url still resolves the
+    dimension because discovery lands on /v1/models."""
+    seen = {}
+
+    def handler(request: httpx.Request) -> httpx.Response:
+        seen["url"] = str(request.url)
+        return httpx.Response(
+            200, json={"data": [{"id": "mistral/mistral-embed", "dimension": 1024}]}
+        )
+
+    _mock_async_client(monkeypatch, handler)
+    provider = GatewayProvider(
+        base_url="http://gw:8083", embedding_model="mistral/mistral-embed"
+    )
+    await provider._detect_dimension()
+    assert provider.get_dimension() == 1024
+    assert seen["url"].endswith("/v1/models")