Move uvicorn import to usage

uvicorn is an optional dependency only used in one function but is imported at the global scope meaning mcp imports will fail if uvicorn is not available.
This commit moves uvicorn import to usage in run_sse_async.
It would be ideal to mark uvicorn as a default-on optional dependency but that isn't supported yet by pyproject.toml.

Author: danlapid

pyproject.toml

@@ -29,7 +29,7 @@ dependencies = [
     "starlette>=0.27",
     "sse-starlette>=1.6.1",
     "pydantic-settings>=2.5.2",
-    "uvicorn>=0.23.1",
+    "uvicorn>=0.23.1; sys_platform != 'emscripten'",
 ]
 
 [project.optional-dependencies]

src/mcp/server/fastmcp/server.py

@@ -15,7 +15,6 @@
 
 import anyio
 import pydantic_core
-import uvicorn
 from pydantic import BaseModel, Field
 from pydantic.networks import AnyUrl
 from pydantic_settings import BaseSettings, SettingsConfigDict
@@ -466,6 +465,7 @@ async def run_stdio_async(self) -> None:
 
     async def run_sse_async(self) -> None:
         """Run the server using SSE transport."""
+        import uvicorn
         starlette_app = self.sse_app()
 
         config = uvicorn.Config(