feat: Antigravity 端点签名迁移至 gemini:chat，流式 usage 提取重构与请求详情自动刷新

- Antigravity 端点签名从 gemini:cli 统一为 gemini:chat，保留向后兼容，含 DB 迁移
- 流式处理中 usage/completion/text 提取抽离为 _update_ctx_from_provider_event，
  支持 envelope 解包后提取，避免格式转换流程中重复计数
- 新增 has_format_conversion 属性，区分真正的格式转换与 envelope rewrite，
  修正 usage 展示层的格式转换标记
- 请求详情抽屉对未完成请求支持自动轮询刷新，关闭时自动停止
- 按次计费样式调整；实际成本仅在倍率非 1.0 时显示；缓存日志降级为 trace

src/services/model/fetch_scheduler.py

@@ -303,7 +303,9 @@ class ModelFetchScheduler:
             # Use request_builder's lazy refresh logic and persist refreshed token back to DB.
             # Endpoint signature is only used for tracing/debug; auth logic doesn't depend on it.
             endpoint_api_format = (
-                "gemini:cli" if prepared.provider_type.lower() == ProviderType.ANTIGRAVITY else None
+                "gemini:chat"
+                if prepared.provider_type.lower() == ProviderType.ANTIGRAVITY
+                else None
             )
             try:
                 resolved = await resolve_oauth_access_token(

src/services/provider/adapters/antigravity/envelope.py

@@ -1,6 +1,6 @@
 """Antigravity v1internal request/response envelope helpers.
 
-Antigravity reuses the `gemini:cli` endpoint signature but wraps the actual
+Antigravity reuses the `gemini:chat` endpoint signature but wraps the actual
 wire format:
 - Request: V1InternalRequest (top-level metadata + nested GeminiRequest)
 - Response: V1InternalResponse (top-level responseId + nested GeminiResponse)

src/services/provider/adapters/antigravity/plugin.py

@@ -249,7 +249,7 @@ async def fetch_models_antigravity(
                 "id": model_id,
                 "owned_by": "antigravity",
                 "display_name": display_name,
-                "api_format": "gemini:cli",
+                "api_format": "gemini:chat",
             }
         )
 
@@ -324,10 +324,14 @@ def register_all() -> None:
     from src.services.provider.transport import register_transport_hook
 
     # Envelope
+    register_envelope("antigravity", "gemini:chat", antigravity_v1internal_envelope)
+    # Backward compat: allow existing endpoints that still use the old signature.
     register_envelope("antigravity", "gemini:cli", antigravity_v1internal_envelope)
     register_envelope("antigravity", "", antigravity_v1internal_envelope)
 
     # Transport
+    register_transport_hook("antigravity", "gemini:chat", build_antigravity_url)
+    # Backward compat: allow existing endpoints that still use the old signature.
     register_transport_hook("antigravity", "gemini:cli", build_antigravity_url)
 
     # Auth