test: fix concurrent start test assertions

[nanookclaw]

Summary

Fixes #581 by removing testing.T assertions from the worker goroutines in TestServer_StartDev_ConcurrentStarts. Each concurrent start now returns an error, and the main test goroutine collects those errors and reports them with require.NoError.

The test still starts 40 dev server instances with a fixed worker pool of 6 concurrent executions. The replacement command helper runs server start-dev with a cancellable context and captured stdout/stderr, so failures from early command exit or cleanup are preserved in the final assertion message without calling FailNow-style methods outside the test goroutine.

Verification

• gofmt -w internal/temporalcli/commands.server_test.go
• git diff --check
• go test ./internal/temporalcli -run '^TestServer_StartDev_ConcurrentStarts$' -count=1

[CLAassistant]

All committers have signed the CLA.