Fix validation extension for operations taking no input

[aaronmallen]

The Validation extension wrapped step methods with |input = {}, *rest, **kwargs, &block| and always forwarded super(validated_input, ...). An operation defining def call with no arguments therefore raised ArgumentError: wrong number of arguments (given 1, expected 0) on call, even when no contract, params, or schema was defined.

Two changes:

• With no contract, there is nothing to validate, so the wrapper now forwards the arguments it was called with untouched instead of manufacturing an empty hash.

• With a contract defined for a method that cannot receive the input, we now raise Dry::Operation::ValidationInputError at class-definition time, whether the contract is declared before or after the method, or inherited by a subclass that defines it. An injected #contract dependency isn't visible then, so the same check also runs on call.

The signature reflection is extracted into a Signature module shared by both paths. It walks past the generic forwarders Dry::Operation prepends to find the method declaring the real signature, which also lets a no-argument call into a keyword-only method pass its validated input as kwargs rather than as a positional hash.

Note that def call(input) with no contract can no longer be invoked as operation.call; it raises ArgumentError, as the signature says it should.