fix: keep shared server alive for a request built before another finishes - #910

Fixes #909

Building a request against the shared server calls serverAddress(), which listens, but state.pending is only incremented in end(). An inner request that finishes first sees pending === 0, closes the server, and deletes the state. The outer end() then dispatches at the closed address. On 7.3.1 that is TypeError: Cannot read properties of null (reading 'address') when server.address() is null. If the URL already contains the port, the same case is ECONNREFUSED.

pending is not incremented at construction. A Test that is built and never sent would otherwise keep the port after every other request finished. When end() finds the shared server it bound already closed, or closing, it listens again on 127.0.0.1 with an ephemeral port, waits for listening, rewrites the URL with the current port, and dispatches. Requests that have already called end() still share one state, and the last one closes the server. A server the caller passed in already listening is left alone.

A Test that is built and never end()ed no longer blocks a later request from closing the server. If nothing in the process calls end(), the server stays listening. That is the existing behavior: construction calls listen(), and only an end() that drops pending to 0 calls close().

Testing

• npx mocha --exit --require should --reporter spec --check-leaks test/supertest.js --grep "shared server late dispatch": before this change, request(server) and request.agent(server) failed with the TypeError (0 passing, 2 failing). After, 4 passing, including the case where the port is already in the URL
• npm test: eslint clean, mocha 141 passing