Add suggestion_commit_message length validation

Fixes http 500 error when trying to set suggestion_commit_message
to value longer than 255 characters using REST API.

Changelog: fixed

app/models/project.rb

@@ -495,6 +495,7 @@ class Project < ApplicationRecord
   validates :variables, nested_attributes_duplicates: { scope: :environment_scope }
   validates :bfg_object_map, file_size: { maximum: :max_attachment_size }
   validates :max_artifacts_size, numericality: { only_integer: true, greater_than: 0, allow_nil: true }
+  validates :suggestion_commit_message, length: { maximum: 255 }
 
   # Scopes
   scope :pending_delete, -> { where(pending_delete: true) }

spec/models/project_spec.rb

@@ -346,6 +346,7 @@ RSpec.describe Project, factory_default: :keep do
     it { is_expected.to validate_presence_of(:namespace) }
     it { is_expected.to validate_presence_of(:repository_storage) }
     it { is_expected.to validate_numericality_of(:max_artifacts_size).only_integer.is_greater_than(0) }
+    it { is_expected.to validate_length_of(:suggestion_commit_message).is_at_most(255) }
 
     it 'validates build timeout constraints' do
       is_expected.to validate_numericality_of(:build_timeout)