Incorporate ParitySharesNamespace and TailPaddingNamespace into IsReserved function

[staheri14]

Proposal

The IsReserved function currently only checks for namespaces that are less than the maximum reserved namespace. However, according to the specifications, the ParitySharesNamespace and TailPaddingNamespace are also considered reserved namespaces. Therefore, conceptually, it is expected that the IsReserved function should take these two namespaces into account when verifying a namespace.

Note that the current implementation ensures that IsParityShares and IsTailPadding are called alongside the IsReserved when needed, however, incorporating these two checks inside the IsReserved function is a more clear approach.

[rootulp]

Agreed, I think it makes sense to have one function (i.e. IsReserved) that determines if a namespace is reserved for protocol use. On a related note I think we should consider renaming

celestia-app/pkg/namespace/consts.go

Lines 54 to 56 in 1b1078c

	// MaxReservedNamespace is lexicographically the largest namespace that is
	// reserved for protocol use.
	MaxReservedNamespace = MustNewV0([]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 255})

because after we implement the change to include tail padding and parity as "reserved" namespaces, the MaxReservedNamespace is not the the largest namespace that is reserved for protocol use.

[staheri14]

@rootulp Could you please provide some insight into the intended usage of the namespace constants (e.g., MaxReservedNamespace, TailPaddingNamespace, and ParitySharesNamespace) declared in the celestia-app/pkg/namespace/consts.go file? Currently, they appear to be utilized exclusively in tests and for verifying whether a namespace is reserved or not. Are there any anticipated external uses for these constants?

Update: TailPaddingNamespace, and ParitySharesNamespace are actually used in other places. So my question is more about MaxReservedNamespace usage.

[rootulp]

They are used outside of tests. I expect they will be use externally by clients but I don't think celestia-app strictly needs to export them as long as they are defined in the specs and external clients can re-define them (if they need them).

MaxReservedNamespace is used to determine if a blob namespace is valid (see here, here). I don't expect other use cases for MaxReservedNamespace.