test: replace subprocess servers with in-process threaded servers for coverage tracking#2341

perhapzz wants to merge 3 commits intomodelcontextprotocol:mainmodelcontextprotocol/python-sdk:mainfrom

perhapzz:fix/1678-streamable-http-test-coverageperhapzz/python-sdk:fix/1678-streamable-http-test-coverageCopy head branch name to clipboard