math/big: Float.GobDecode accepts an invalid form that panics in Int64, Float64, Int, and Rat #81695

Go version

go version go1.27.1 darwin/arm64 (also reproduced on tip at a532f49)

What did you do?

var f big.Float
fmt.Println(f.GobDecode([]byte{1, 6, 0, 0, 0, 64})) // form bits = 3
v, acc := f.Int64()
fmt.Println(v, acc)

The same bytes decode the same way through a gob.Decoder reading from a stream.

What did you see happen?

GobDecode returns nil, and Int64, Float64, Int and Rat panic with unreachable.
The decoded value is inconsistent in other ways too: String returns "0" while Sign returns 1.

What did you expect to see?

An error from GobDecode. The form field has three valid values (zero, finite, inf), and validate0, added for #57946, only checks the mantissa and precision of a finite value.
Int.GobDecode (#81490) and Rat.GobDecode (#81645) were fixed recently for the same kind of state that decodes without an error and panics later.