cmd/server: SIGINT during initialization can cause a data race in ApiServer

I ran:

; go run -race ./cmd/server
<first log line>
<ctrl-c (SIGINT)>

and then saw:

; go run -trimpath -race ./cmd/server
# github.com/Shopify/toxiproxy/v2/cmd/server
ld: warning: '/private/var/folders/db/svmm3t1x3yn4d1skpbq3ddv00000gn/T/go-link-1052203798/000013.o' has malformed LC_DYSYMTAB, expected 98 undefined symbols to start at index 1626, found 95 undefined symbols starting at index 1626
{"level":"info","version":"git","caller":"server.go:78","time":"2025-09-04T02:46:27Z","message":"Starting Toxiproxy"}
{"level":"info","address":"localhost:8474","caller":"api.go:57","time":"2025-09-04T02:46:27Z","message":"Starting Toxiproxy HTTP server"}
^C{"level":"info","caller":"server.go:104","time":"2025-09-04T02:46:38Z","message":"Shutdown started"}
==================
WARNING: DATA RACE
Read at 0x00c00028a6d8 by main goroutine:
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Shutdown()
      github.com/Shopify/toxiproxy/v2/api.go:76 +0x3c
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:105 +0xba0
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20

Previous write at 0x00c00028a6d8 by goroutine 11:
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Listen()
      github.com/Shopify/toxiproxy/v2/api.go:59 +0x1c0
  main.run.func1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:95 +0x48
  main.run.gowrap1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:99 +0x94

Goroutine 11 (running) created at:
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:94 +0xac8
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20
==================
==================
WARNING: DATA RACE
Write at 0x00c000406090 by main goroutine:
  ??()
      -:0 +0x1031e61d8
  sync/atomic.StoreUint32()
      <autogenerated>:1 +0x14
  net/http.(*Server).Shutdown()
      net/http/server.go:3180 +0x48
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Shutdown()
      github.com/Shopify/toxiproxy/v2/api.go:83 +0x98
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:105 +0xba0
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20

Previous write at 0x00c000406090 by goroutine 11:
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Listen()
      github.com/Shopify/toxiproxy/v2/api.go:59 +0xa4
  main.run.func1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:95 +0x48
  main.run.gowrap1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:99 +0x94

Goroutine 11 (running) created at:
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:94 +0xac8
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20
==================
==================
WARNING: DATA RACE
Write at 0x00c0004060b8 by main goroutine:
  ??()
      -:0 +0x1031eb900
  sync/atomic.CompareAndSwapInt32()
      <autogenerated>:1 +0x14
  sync.(*Mutex).Lock()
      sync/mutex.go:46 +0x2c
  net/http.(*Server).Shutdown()
      net/http/server.go:3182 +0x58
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Shutdown()
      github.com/Shopify/toxiproxy/v2/api.go:83 +0x98
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:105 +0xba0
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20

Previous write at 0x00c0004060b8 by goroutine 11:
  github.com/Shopify/toxiproxy/v2.(*ApiServer).Listen()
      github.com/Shopify/toxiproxy/v2/api.go:59 +0xa4
  main.run.func1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:95 +0x48
  main.run.gowrap1()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:99 +0x94

Goroutine 11 (running) created at:
  main.run()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:94 +0xac8
  main.main()
      github.com/Shopify/toxiproxy/v2/cmd/server/server.go:54 +0x20
==================
Found 3 data race(s)
exit status 66

This is caused due to unsynchronized access to the underlying *http.Server held on *ApiServer.
A follow-up PR coming up with a fix.

Start with api.go at the ApiServer.Listen and Shutdown paths, then inspect cmd/server/server.go where initialization and SIGINT shutdown run concurrently. Reproduce the issue with go run -race ./cmd/server and verify that interrupting during initialization no longer reports races.