Server url truncated by urllib in FastMCP #200

Describe the bug
When using urllib url joining, this truncates server urls paths when combining /messages/ set in the transport object.

sse = SseServerTransport("/messages/")

For example, if joining http://localhost:8000/some/path/to/sse with /messages/, it will output: http://localhost:8000/messages/ instead of honoring the path.

Removing the leading slash fixes this.

This was found in the situation where my server was being proxied and accessed at a subpath. afaict, there is no explicit reason to keep the leading slash. This is also something that could be addressed in #194 since a dev could then customize the server setup more easily.

To Reproduce

from urllib.parse import urljoin

url = "http://localhost:8000/some/path/to/sse"
sse_data = "/messages/?session_id=123"

endpoint_url = urljoin(url, sse_data)
assert endpoint_url == "http://localhost:8000/messages/?session_id=123"

####

url = "http://localhost:8000/some/path/to/sse"
sse_data = "messages/?session_id=123"

endpoint_url = urljoin(url, sse_data)
assert endpoint_url == "http://localhost:8000/some/path/to/messages/?session_id=123"

Expected behavior
A clear and concise description of what you expected to happen.
I would expect this to join the route to the end of the url instead of truncating it.