fix(rerank): validate credentials for explicitly configured providers (#4443)

* fix(rerank): validate credentials for explicitly configured providers

RerankConfig checked required fields for openai and litellm only. An explicit
`provider: cohere` without api_key, or `provider: vikingdb` without ak/sk, was
accepted at load time. is_available() then returned False and
HierarchicalRetriever fell back to plain vector search, logging a single
info-level line saying rerank was not configured.

The new checks run against the effective provider, matching the existing
openai and litellm branches. Auto-detection is unaffected, since detecting
cohere already requires api_key and detecting vikingdb already requires ak and
sk. An empty RerankConfig() still resolves to no provider and stays valid.

Drops test_default_provider_is_vikingdb, which asserted a default that
auto-detection replaced and had been failing on main. Rewrites
test_unknown_provider_raises_value_error to actually cover an unknown provider
and adds coverage for the two providers that were missing validation.

* docs(configuration): state required credentials per rerank provider

The rerank section described credential inference but not the fields each
provider requires when provider is set explicitly.

---------

Co-authored-by: Terminator666666 <Terminator666666@users.noreply.github.com>

Author: Terminator666666

docs/en/configuration/01-server.md

@@ -147,6 +147,8 @@ Changing the model or `dimension` can make existing vector collections incompati
 
 Rerank has no separate `enabled` field. It becomes available when the required provider credentials are configured.
 
+Setting `provider` explicitly requires the credentials that provider needs: `ak` and `sk` for `vikingdb`, `api_key` for `cohere`, `api_key` and `api_base` for `openai`, `model` for `litellm`. An incomplete block is rejected when the configuration loads.
+
 ## Retrieval Settings
 
 ```json

docs/zh/configuration/01-server.md

@@ -147,6 +147,8 @@ API 型 `embedding`、`vlm`、`query_planner` 和 `rerank` 配置会复用部分
 
 Rerank 没有单独的 `enabled` 字段；配置了对应 provider 所需的凭证后才会启用。
 
+显式指定 `provider` 时必须提供该 provider 所需的凭证：`vikingdb` 需要 `ak` 和 `sk`，`cohere` 需要 `api_key`，`openai` 需要 `api_key` 和 `api_base`，`litellm` 需要 `model`。凭证不全的配置在加载时即被拒绝。
+
 ## 检索配置
 
 ```json

openviking_cli/utils/config/rerank_config.py

@@ -88,6 +88,12 @@ def validate_provider_fields(self) -> "RerankConfig":
         if provider == "litellm":
             if not self.model:
                 raise ValueError("LiteLLM rerank provider requires 'model'")
+        if provider == "cohere":
+            if not self.api_key:
+                raise ValueError("Cohere rerank provider requires 'api_key'")
+        if provider == "vikingdb":
+            if not self.ak or not self.sk:
+                raise ValueError("VikingDB rerank provider requires 'ak' and 'sk'")
         return self
 
     def is_available(self) -> bool:

tests/misc/test_rerank_openai.py

@@ -268,10 +268,13 @@ def test_openai_requires_api_key_and_api_base(self):
         with pytest.raises(ValidationError):
             RerankConfig(provider="openai", api_base="https://example.com/rerank")
 
-    def test_default_provider_is_vikingdb(self):
-        config = RerankConfig()
-        assert config.provider == "vikingdb"
-
     def test_unknown_provider_raises_value_error(self):
-        with pytest.raises(ValueError, match="provider"):
+        with pytest.raises(ValidationError, match="provider"):
+            RerankConfig(provider="unknown", api_key="key")
+
+    def test_explicit_provider_requires_its_own_credentials(self):
+        with pytest.raises(ValidationError, match="api_key"):
             RerankConfig(provider="cohere", ak="ak", sk="sk")
+
+        with pytest.raises(ValidationError, match="ak"):
+            RerankConfig(provider="vikingdb", api_key="key")