i can't find reference to this in the spec, can you provide a link?

Thank you for pointing this out. I initially thought the official MCP specification mandated a specific path for the SSE message endpoint, but after rechecking, it turns out it does not. According to the official MCP specification, servers should send an endpoint event to inform clients of the URI to use for posting messages.

The motivation for this PR is to align with common practice observed in the MCP ecosystem, where /messages is frequently used for SSE message endpoints:

• MCP Official TypeScript SDK examples - Uses /messages in SSE examples
• MCP Official Python SDK - SseServerTransport constructor accepts an endpoint parameter, commonly set to /messages

It improves out-of-the-box interoperability and reduces confusion.

Since SSE is already a deprecated transport in the MCP spec, if keeping /mcp/message is preferred for Spring AI conventions or backward compatibility, I'm happy to close this PR or discuss alternatives.