[wpilibc] Remove Alert magic static with map lookup

[virtuald]

The magic static adds yet another thing that needs to be reset if you want to run a unit test with a completely reset wpilib state. Use the existing Sendable static map to store the data instead.

This leaks memory if ResetSmartDashboardInstance is called.

Alternative to #7711

[PeterJohnson]

I'd prefer to figure out a global fix using ManagedStatic or similar for 2027, as this approach feels like a hack that we'd need to back out for Sendable changes. Is there a compelling reason to make this change for 2025?

[virtuald]

Well, I tried to convert the C++ tests to python and every test after the first one fails, because we restart NT each time a new test occurs (unlike the C++ tests).

If anyone tries to use Alerts in their robotpy robot tests it will fail after the first time because robotpy resets all the reset of the wpilib state.

An alternative would be to add a frc::impl::ResetAlerts function to clear the static variable, adding yet another reset function. It would be more invasive because frc::alert holds a pointer to the set inside the static, so those would need to be converted to shared_ptr in addition to adding the reset function.