/message break existing usage

[Yuan325]

Describe the bug
A clear and concise description of what the bug is.
This PR (5a25fe3) was recently submitted and break our MCP server's usage with the Inspector. Instead of using the endpoint that was returned (e.g. http://127.0.0.1:5000/mcp?sessionId=123) from /sse endpoint, it updates the endpoint with /messages (http://127.0.0.1:5000/mcp/messages?sessionId=123. This rule was not clearly specified in the mcp docs/specs (only showed in an example).

To Reproduce
Steps to reproduce the behavior:

1. MCP server with /sse endpoint for GET and / endpoint for POST.

Expected behavior
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Client should use the endpoint received from GET.

Logs
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n/a

Additional context
Add any other context about the problem here.

If this is expected, it will be great to include it within the docs/specifications.