Avoid duplicate request handling after server restarts

[xiehuanyi]

Closing a Polka server and calling .listen() again attaches the same request handler twice. An asynchronous route then runs twice for one HTTP request, duplicating its work.

Attach the handler only when it is not already registered on the server. Add a real HTTP restart regression that checks one route invocation, a valid response, and preservation of a separate request listener.

Validation: the regression records two route invocations before the fix (345 assertions pass, one fails). The complete Tape suite passes all 346 assertions on Node 8.17, 10.24, and 12.22, run sequentially to avoid the existing shared test output file. git diff --check passes. This branch is based on main and does not include #228.

AI disclosure: OpenAI Codex assisted with investigation, implementation, and validation.

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