Show session id per request in simpleStreamableHttp example server

[cliffhall]

• In src/examples/server/simpleStreamableHttp.ts
  • In mcpPostHandler,
    • Get mcp-session-id header early so that it can be reported in every incoming request.
    • Helpful for troubleshooting Inspector's ability to retain the session id for the Proxy <-> Server leg
  • Linter fixed a few small whitespace foibles.

Motivation and Context

• A recent issue in the Inspector repo questioned the its proxy's ability to retain and send the appropriate mcp-session-id header to the server.
• In order to demonstrate that each request following the initialize request carried this header with the appropriate id.
• This is a go-to, in-project server that we frequently test simple streamable connections as well as OAuth.
• The only problem was that this server did not report the session id for each incoming request, because it did not extract the header before reporting the request and its contents.
• This change simply gets the header as soon as the request comes in and if the sessionId is present, it includes it in its log output.

How Has This Been Tested?

• See: Local Proxy Causes Session ID Mismatch with Streamable HTTP Servers inspector#492 (comment)

Breaking Changes

Nope.

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
• I have added or updated documentation as needed

Additional context

[ihrpr]

Thank you!