Bug: request_meta is not available in server.request_context #103

Describe the bug
This makes it impossible to build a server with progress notifications

To Reproduce
Steps to reproduce the behavior:
do a tool_call for example:

{
"method": "tools/call",
"params": {
"name": "6a41a978-0a31-4eaf-9d0c-431a5f784bc9",
"arguments": {
"input_value": "3*2"
},
"_meta": {
"progressToken": 0
}
}
}

server.request_context.meta is None

Expected behavior
meta should include the progressToken

It's not getting assigned here https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/__init__.py#L447