How much plausible deniability would Iran have if they gave a nuke to Hezbollah who fired it over the border at Tel Aviv?

"That was Hezbollah, not us!"

You might say using a proxy would be a hopelessly transparent ploy, but Hezbollah has been firing other Iranian supplied weapons at Israel for years and yet many people swear up and down that Iran has "never attacked Israel". So apparently the proxy ploy does work on a lot of people.