When endpoints are declared after calling `add_mcp_server`, they are not registered by the MCP server

Simply put, this example would work:
```python
@app.get("/items/")
async def list_items():
    return ["item 1", "item 2", "item 3"]

add_mcp_server(
    app,
    mount_path="/mcp",
)
```

This won't:
```python
add_mcp_server(
    app,
    mount_path="/mcp",
)

@app.get("/items/")
async def list_items():
    return ["item 1", "item 2", "item 3"]
```


Started from this thread: #18 
