[ipv4] fix Cidr::Clear() (#11879)

The `Cidr` class was incorrectly inheriting from `Clearable<Address>`
instead of `Clearable<Cidr>`. This oversight meant that calling
`Clear()` on a `Cidr` object would not clear its `mLength` member,
only the `mAddress` field. This change corrects the template
parameter for the `Clearable` base class to `Cidr`, ensuring the
entire object is properly zeroed out.

This bug could cause issues in the NAT64 translator. When a user calls
`otNat64ClearIp4Cidr()` to disable translation, the underlying
`mIp4Cidr.Clear()` method would fail to clear the CIDR length. The
`Nat64::Translator` would check `mIp4Cidr.mLength > 0` in its
`UpdateState()` method to determine if a valid CIDR is configured.
Because `mLength` was not cleared, this check would pass incorrectly,
causing NAT64 translation to continue with an invalid,
partially-cleared CIDR instead of stopping as expected.

src/core/net/ip4_types.hpp

@@ -189,7 +189,7 @@ private:
 /**
  * Represents an IPv4 CIDR block.
  */
-class Cidr : public otIp4Cidr, public Unequatable<Cidr>, public Clearable<Address>
+class Cidr : public otIp4Cidr, public Unequatable<Cidr>, public Clearable<Cidr>
 {
     friend class Address;