feat(vertex-ai): 重构 Vertex AI 为插件化 adapter，支持 service_account 认证与动态路由

将 Vertex AI 从 transport.py 的硬编码逻辑重构为独立的 plugin adapter，
支持 service_account/oauth 认证类型、模型格式自动识别、区域路由和 URL 构建。
前端新增 Key 认证类型选择和 Service Account 配置表单。

Co-authored-by: NyaDoo <65238336+NyaDoo@users.noreply.github.com>
Closes #194

tests/services/test_upstream_fetcher_vertex_ai_models.py

@@ -0,0 +1,150 @@
+from __future__ import annotations
+
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from src.core.vertex_auth import VertexAuthService
+from src.services.model.upstream_fetcher import (
+    EndpointFetchConfig,
+    UpstreamModelsFetchContext,
+    fetch_models_for_key,
+)
+
+
+@pytest.mark.asyncio
+async def test_fetch_models_for_key_vertex_api_key_custom_fetcher() -> None:
+    ctx = UpstreamModelsFetchContext(
+        provider_type="vertex_ai",
+        api_key_value="test-api-key",
+        format_to_endpoint={
+            "gemini:chat": EndpointFetchConfig(base_url="https://aiplatform.googleapis.com"),
+        },
+        proxy_config=None,
+        auth_config={},
+    )
+
+    mocked_models = [
+        {
+            "id": "gemini-2.5-pro",
+            "owned_by": "google",
+            "display_name": "Gemini 2.5 Pro",
+            "api_format": "gemini:chat",
+        }
+    ]
+
+    with (
+        patch(
+            "src.services.provider.adapters.vertex_ai.plugin._fetch_models_from_url",
+            AsyncMock(return_value=(mocked_models, None, True)),
+        ),
+        patch(
+            "src.services.proxy_node.resolver.build_proxy_client_kwargs",
+            return_value={"timeout": 1.0},
+        ),
+    ):
+        models, errors, ok, meta = await fetch_models_for_key(ctx, timeout_seconds=1.0)
+
+    assert ok is True
+    assert errors == []
+    assert meta is None
+    assert [m.get("id") for m in models] == ["gemini-2.5-pro"]
+
+
+@pytest.mark.asyncio
+async def test_fetch_models_for_key_vertex_service_account_ignores_soft_404_when_success() -> None:
+    auth_config = {
+        "project_id": "demo-project",
+        "client_email": "svc@example.iam.gserviceaccount.com",
+        "private_key": "-----BEGIN PRIVATE KEY-----\nTEST\n-----END PRIVATE KEY-----\n",
+        "region": "global",
+    }
+    ctx = UpstreamModelsFetchContext(
+        provider_type="vertex_ai",
+        api_key_value="__placeholder__",
+        format_to_endpoint={
+            "gemini:chat": EndpointFetchConfig(base_url="https://aiplatform.googleapis.com"),
+            "claude:chat": EndpointFetchConfig(base_url="https://aiplatform.googleapis.com"),
+        },
+        proxy_config=None,
+        auth_config=auth_config,
+    )
+
+    fetch_side_effect = [
+        (
+            [
+                {
+                    "id": "gemini-2.0-flash",
+                    "owned_by": "google",
+                    "display_name": "Gemini 2.0 Flash",
+                    "api_format": "gemini:chat",
+                }
+            ],
+            None,
+            True,
+        ),
+        ([], "HTTP 404: not found", False),
+    ]
+
+    with (
+        patch.object(
+            VertexAuthService,
+            "get_access_token",
+            AsyncMock(return_value="ya29.test-token"),
+        ),
+        patch(
+            "src.services.provider.adapters.vertex_ai.plugin._iter_regions",
+            return_value=["global"],
+        ),
+        patch(
+            "src.services.provider.adapters.vertex_ai.plugin._fetch_models_from_url",
+            AsyncMock(side_effect=fetch_side_effect),
+        ),
+        patch(
+            "src.services.proxy_node.resolver.build_proxy_client_kwargs",
+            return_value={"timeout": 1.0},
+        ),
+    ):
+        models, errors, ok, meta = await fetch_models_for_key(ctx, timeout_seconds=1.0)
+
+    assert ok is True
+    assert errors == []
+    assert meta is None
+    ids = {m.get("id") for m in models}
+    assert "gemini-2.0-flash" in ids
+
+
+@pytest.mark.asyncio
+async def test_fetch_models_for_key_vertex_api_key_returns_soft_404_when_all_failed() -> None:
+    ctx = UpstreamModelsFetchContext(
+        provider_type="vertex_ai",
+        api_key_value="test-api-key",
+        format_to_endpoint={
+            "gemini:chat": EndpointFetchConfig(base_url="https://aiplatform.googleapis.com"),
+        },
+        proxy_config=None,
+        auth_config={},
+    )
+
+    with (
+        patch(
+            "src.services.provider.adapters.vertex_ai.plugin._fetch_models_from_url",
+            AsyncMock(
+                side_effect=[
+                    ([], "HTTP 404: not found", False),
+                    ([], "HTTP 404: not found", False),
+                ]
+            ),
+        ),
+        patch(
+            "src.services.proxy_node.resolver.build_proxy_client_kwargs",
+            return_value={"timeout": 1.0},
+        ),
+    ):
+        models, errors, ok, meta = await fetch_models_for_key(ctx, timeout_seconds=1.0)
+
+    assert ok is False
+    assert models == []
+    assert meta is None
+    assert errors
+    assert "HTTP 404" in errors[0]