Fix type signature for ActiveRecord::Base.validate (to better support official Ruby documentation) (#859)

* Add unit test for #874

* Backport fix from https://github.com/ruby/gem_rbs_collection/blob/7337c95459850e31a8af86e26d369b7b4c04a91c/gems/activerecord/6.0/activerecord.rbs#L3

Author: yuzisee

gems/activemodel/6.0/activemodel.rbs

@@ -46,7 +46,7 @@ module ActiveModel
     attr_accessor validation_context: untyped
 
     module ClassMethods
-      type condition[T] = Symbol | ^(T) [self: T] -> boolish
+      type condition[T] = Symbol | ^(T) [self: T] -> boolish | Proc
       type conditions[T] = condition[T] | Array[condition[T]]
 
       # Validates each attribute against a block.

gems/activemodel/7.1/activemodel.rbs

@@ -46,7 +46,7 @@ module ActiveModel
     attr_accessor validation_context: untyped
 
     module ClassMethods
-      type condition[T] = Symbol | ^(T) [self: T] -> boolish
+      type condition[T] = Symbol | ^(T) [self: T] -> boolish | Proc
       type conditions[T] = condition[T] | Array[condition[T]]
 
       # Validates each attribute against a block.

gems/activerecord/7.2/_test/activerecord.rb

@@ -7,6 +7,36 @@ class TestCallbackObject < ActiveRecord::Base
   after_save ::Callbacks::ClassCallback
   around_create ::Callbacks::ClassCallback
   around_save ::Callbacks::ClassCallback
+
+  def local_condition1
+    [true, false].sample
+  end
+
+  def local_condition2
+    [true, false].sample
+  end
+
+  def local_condition3
+    [true, false].sample
+  end
+
+  # https://guides.rubyonrails.org/v7.2/active_record_validations.html#combining-validation-conditions
+  RAILS_72_DOCS_EXAMPLE = [
+    :local_condition1,
+    ->(my_rec) { my_rec.local_condition2 },
+    Proc.new do |my_rec|
+      # @type var my_rec: TestCallbackObject
+      my_rec.local_condition3
+    end
+  ]
+
+  validate :custom_validation, on: :update, unless: RAILS_72_DOCS_EXAMPLE
+
+  def custom_validation
+    if self.record_timestamps?
+      self.errors.add(:base, 'testing only')
+    end
+  end
 end
 
 module Callbacks

gems/activerecord/7.2/_test/activerecord.rbs

@@ -1,4 +1,11 @@
 class TestCallbackObject < ActiveRecord::Base
+  def local_condition1: () -> bool
+  def local_condition2: () -> bool
+  def local_condition3: () -> bool
+
+  RAILS_72_DOCS_EXAMPLE: [Symbol, ^(TestCallbackObject) [self: TestCallbackObject] -> bool, Proc]
+
+  def custom_validation: () -> void
 end
 
 module Callbacks