`HyperServerOptions::custom_streamable_http_endpoint` is not properly used

[jsudano]

Describe the bug
I noticed while trying to configure a Streamable HTTP MCP server using HyperServerOptions that custom_streamable_http_endpoint doesn't seem to work. While digging through the code I noticed HyperServerOptions impl's pub fn streamable_http_endpoint instead returns self.custom_messages_endpoint

I'll open a PR with a fix shortly.

What are you using rust-mcp-sdk for?
Select all that apply:

• Developing a MCP Server
• Developing a MCP Client

Technical details:

• rust-mcp-sdk Version : 0.7.3
• Cargo features enabled for rust-mcp-sdk : ["server", "macros", "streamable-http", "sse", "hyper-server"]
• Environment/OS (Mac, Linux, Windows, etc) : Mac, Linux

To Reproduce
Steps to reproduce the behavior:

1. Starting with the streamable http hello world example, add a custom_streamable_http_endpoint value to the HyperServerOptions in main.rs
2. Start the server, observe in the server logs that the server still says it's listening for Streamable HTTP connections on <base_url>/mcp
3. Confirm with a Streamable HTTP client of your choosing that connections to your custom endpoint are rejected

Expected behavior
The server properly listens for streamable HTTP connections on the provided endpoint