refactor(core): use constant for server install command to reduce duplication

Extract the repeated install command string into a constant _SERVER_INSTALL_CMD
to make the code more maintainable and easier to update in the future.

This addresses code review feedback about reducing duplication in the package
mapping structure.

Co-authored-by: OhYee <[email protected]>

Author: Copilot

agentrun/__init__.py

@@ -292,10 +292,11 @@
 # 可选依赖包映射：导入错误的包名 -> 安装使用的包名
 # Optional dependency mapping: import error package name -> installation package name
 # 所有 server 相关的可选依赖 / All server-related optional dependencies
+_SERVER_INSTALL_CMD = "agentrun-sdk[server]"
 _OPTIONAL_PACKAGES = [
-    ("fastapi", "agentrun-sdk[server]"),
-    ("uvicorn", "agentrun-sdk[server]"),
-    ("ag_ui", "agentrun-sdk[server]"),
+    ("fastapi", _SERVER_INSTALL_CMD),
+    ("uvicorn", _SERVER_INSTALL_CMD),
+    ("ag_ui", _SERVER_INSTALL_CMD),
 ]