Add tests and mark internal classes as nodoc

Test that each differing method appears once in failure_message (the
diff array contains a method twice when it differs between sides, and
`.uniq` prevents duplicate blocks in the output). Also test that
assert_interfaces_match passes for matching pairs even when another
class in the list mismatches.

Mark ParamsNormalizer, NullParamsNormalizer, and MethodInspector as
:nodoc: since they are internal implementation details.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

lib/duck_typer/method_inspector.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module DuckTyper
-  module MethodInspector
+  module MethodInspector # :nodoc:
     TYPES = %i[instance_methods class_methods].freeze
 
     def self.for(object, type)

lib/duck_typer/null_params_normalizer.rb

@@ -4,7 +4,7 @@ module DuckTyper
   # A no-op params processor that returns params unchanged. Used when
   # interface comparison should preserve original parameter names rather
   # than normalizing them.
-  class NullParamsNormalizer
+  class NullParamsNormalizer # :nodoc:
     def self.call(params)
       params
     end

lib/duck_typer/params_normalizer.rb

@@ -6,7 +6,7 @@ module DuckTyper
   # are equivalent — so keywords are sorted alphabetically. Positional
   # argument names are also replaced with sequential placeholders,
   # focusing the comparison on parameter structure rather than naming.
-  class ParamsNormalizer
+  class ParamsNormalizer # :nodoc:
     KEYWORD_TYPES = %i[key keyreq].freeze
     SEQUENTIAL_TYPES = %i[req opt rest keyrest block].freeze
 

test/duck_typer/interface_checker_test.rb

@@ -439,6 +439,15 @@ def test_failure_message_includes_differing_method_signatures
     assert_includes message, "foo()"
   end
 
+  def test_failure_message_shows_each_differing_method_once
+    left = Class.new { def foo(a) = nil }
+    right = Class.new { def foo = nil }
+
+    message = call_checker(left, right).failure_message
+
+    assert_equal 1, message.scan("foo(a)").size
+  end
+
   def test_failure_message_shows_not_defined_for_missing_method
     left = Class.new { def foo = nil; def bar = nil }
     right = Class.new { def foo = nil }

test/duck_typer/minitest_test.rb

@@ -12,6 +12,15 @@ def test_passes_when_interfaces_match
     assert_interfaces_match [a, b, c]
   end
 
+  def test_passes_for_matching_pairs_even_when_another_class_mismatches
+    a = Class.new { def foo(a) = nil }
+    b = Class.new { def foo(a) = nil }
+    c = Class.new { def foo = nil }
+
+    assert_interfaces_match [a, b]
+    assert_raises(Minitest::Assertion) { assert_interfaces_match [a, b, c] }
+  end
+
   def test_fails_when_interfaces_do_not_match
     a = Class.new { def foo(a) = nil }
     b = Class.new { def foo(a) = nil }