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

src/core/provider_templates/fixed_providers.py

@@ -72,7 +72,7 @@ FIXED_PROVIDERS: dict[ProviderType, FixedProviderTemplate] = {
         provider_type=ProviderType.CODEX,
         display_name="Codex",
         api_base_url="https://chatgpt.com/backend-api/codex",
-        endpoint_signatures=["openai:cli"],
+        endpoint_signatures=["openai:cli", "openai:compact"],
         oauth=FixedProviderOAuth(
             authorize_url="https://auth.openai.com/oauth/authorize",
             token_url="https://auth.openai.com/oauth/token",
@@ -121,6 +121,23 @@ FIXED_PROVIDERS: dict[ProviderType, FixedProviderTemplate] = {
             use_pkce=False,
         ),
     ),
+    ProviderType.VERTEX_AI: FixedProviderTemplate(
+        provider_type=ProviderType.VERTEX_AI,
+        display_name="Vertex AI",
+        # Vertex uses fixed global base URL; concrete upstream path is selected by transport hook.
+        api_base_url="https://aiplatform.googleapis.com",
+        endpoint_signatures=["gemini:chat", "claude:chat"],
+        # Vertex does not use this OAuth flow (it uses API Key / Service Account).
+        oauth=FixedProviderOAuth(
+            authorize_url="",
+            token_url="",
+            client_id="",
+            client_secret="",
+            scopes=[],
+            redirect_uri="",
+            use_pkce=False,
+        ),
+    ),
     ProviderType.ANTIGRAVITY: FixedProviderTemplate(
         provider_type=ProviderType.ANTIGRAVITY,
         display_name="Antigravity",

src/core/provider_types.py

@@ -21,6 +21,7 @@ class ProviderType(str, Enum):
     CODEX = "codex"
     GEMINI_CLI = "gemini_cli"
     ANTIGRAVITY = "antigravity"
+    VERTEX_AI = "vertex_ai"
 
 
 # 所有有效 provider_type 值的集合（用于校验）