Fix panic running plugin subcommands with nil context

[xheggs]

Summary

Every executed plugin subcommand under stripe projects (init, status, catalog, search, add, …) panics with cannot create context from nil parent instead of running, making the Projects plugin unusable.

runPluginCmd passes cmd.Context() into withSIGTERMCancel, but cmd.Context() is nil for the dynamically-registered plugin subcommand stubs, so context.WithCancel(nil) panics under Go 1.15+.

The help path in the same file already guards against this with SetContext(context.Background()); this PR applies the same guard to the execution path.

Fixes #1707.

Change

parentCtx := cmd.Context()
if parentCtx == nil {
	parentCtx = context.Background()
}
ctx := withSIGTERMCancel(parentCtx, func() { ... })

Reproduction

stripe plugin install projects        # v0.20.0
stripe login
stripe projects status                # panics

Stack trace:

panic: cannot create context from nil parent
context.WithCancel({0x0?, 0x0?})
github.com/stripe/stripe-cli/pkg/cmd.withSIGTERMCancel(...)   pkg/cmd/listen.go:223
github.com/stripe/stripe-cli/pkg/cmd.(*pluginTemplateCmd).runPluginCmd(...)   pkg/cmd/plugin_cmds.go:105
github.com/stripe/stripe-cli/pkg/cmd.addPluginSubcommandStubs.func1(...)   pkg/cmd/plugin_cmds.go:91

Testing

• Reproduced the panic on v1.42.15 (Homebrew) and on a clean build of the tag.
• Built from source with this change: stripe projects status / init / catalog now run normally instead of panicking.
• go build ./... and gofmt clean.

Environment

• stripe-cli v1.42.15 (also reproduced on v1.40.8)
• projects plugin v0.20.0
• macOS 26.2, arm64

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