Reproduce Repository X-Ray functionality - Introduce lock file classes

What does this MR do and why?

Context

In #474306 (comment 2025085630), we decided to migrate the Repository X-Ray functionality into the GitLab Rails monolith. This gives us two main benefits: (i) it will eventually allow us to run the service outside of the CI pipeline, and (ii) we can maintain the parsing logic centrally so that other domains can utilize it.

This MR

This MR is the first step in the migration progress. It introduces the LockFiles::Base class where the intention is for each lock file type to be represented by a child class. The RubyGems child class serves as an example of the implementation.

Next Steps: Eventually we will add all the languages/lock files currently supported in Repository X-Ray. Then we will introduce a lock file parser class that utilizes these LockFiles::Base child classes.

POC: For a more complete picture of the overall implementation, see: Draft: POC - Reproduce base functionality of Re... (!162125 - closed)

Resolves part of #476177 (closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #476177 (closed)

ee/lib/ai/context/dependencies/lock_files/base.rb

+# frozen_string_literal: true
+
+module Ai
+  module Context
+    module Dependencies
+      module LockFiles
+        # This class represents a dependency manager lock file. It contains logic
+        # to parse and extract libraries from the file content. To support a new
+        # lock file type/language, create a new class that inherits from this Base.
+        # Then append the new class name to LockFiles::Constants::LOCK_FILE_CLASSES.
+        class Base
+          include Gitlab::Utils::StrongMemoize
+
+          # This value is supposed to indicate the version of Repository X-Ray we are using;
+          # however, it's not applicable here so we just set it to a placeholder value.
+          # It may be removed entirely in https://gitlab.com/gitlab-org/gitlab/-/issues/479185.
+          SCANNER_VERSION = '0.0.0'
+
+          Lib = Struct.new(:name, :version, keyword_init: true)
+
+          def self.file_name_glob
+            raise NotImplementedError
+          end
+
+          def self.lang_name
+            raise NotImplementedError
+          end
+
+          def self.lang
+            CodeSuggestions::ProgrammingLanguage::LANGUAGE_XRAY_NAMING[lang_name]
+          end
+
+          def self.matches?(path)
+            File.fnmatch?("**/#{file_name_glob}", path, File::FNM_PATHNAME | File::FNM_DOTMATCH)
+          end
+
+          def initialize(blob)
+            @content = blob.data
+            @path = blob.path
+            @errors = []
+          end
+
+          def parse!
+            return error('file empty') if content.blank?
+
+            @libs = extract_libs
+
+            # Default error message if there are no other errors
+            error('format not recognized or dependencies not present') if libs.blank? && errors.empty?
+          end
+
+          # This hash matches the current XrayReport payload schema
+          def payload
+            return unless libs.present?
+
+            {
+              libs: formatted_libs,
+              checksum: checksum,
+              fileName: path,
+              scannerVersion: SCANNER_VERSION
+            }
+          end
+
+          def valid?
+            errors.empty?
+          end
+
+          def error_message
+            return if valid?
+
+            "Error(s) while parsing file `#{path}`: #{errors.join(', ')}"
+          end
+
+          private
+
+          attr_reader :content, :path, :libs, :errors
+
+          def extract_libs
+            raise NotImplementedError
+          end
+
+          def formatted_libs
+            libs.map do |lib|
+              lib_name = lib.version ? "#{lib.name} (#{lib.version})" : lib.name
+
+              { name: lib_name }
+            end
+          end
+
+          def error(message)
+            @errors << message
+          end
+
+          def checksum
+            Digest::SHA256.hexdigest(content)
+          end
+          strong_memoize_attr :checksum
+        end
+      end
+    end
+  end
+end