fix: normalize headers in sse transport

[marcrasi]

The SSE transport does not properly forward custom headers that are provided as a Headers object or as a [string, string][].

The Streamable HTTP transport already implements the correct solution to this problem, so I moved the solution into a common helper function, used it from both transports, and added a test to the SSE transport that exercises this.

Motivation and Context

Bug

How Has This Been Tested?

I have an internal application that passes custom headers [["Authorization", "Bearer XYZ"]] to the client. Before this change, this was getting forwarded to the server as a header {0: "Authorization,Bearer XYZ"}. After this change, it is correctly forwarded as {Authorization: "Bearer XYZ"}.

Also, I added a unit test.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling (N/A: doesn't add new error possibilities)
• I have added or updated documentation as needed (N/A: just a fix to existing functionality)

Additional context

None