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fix: cover h2c connections in graceful shutdown and prove the drain

Share the http2.Server between the h2c handler and http2.ConfigureServer
so Shutdown sends GOAWAY to hijacked h2c connections. Extract the
duplicated shutdown goroutine into a gracefulShutdown helper that skips
its completion log when the server never started, and add a slow-handler
test that fails without the drain wait.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>