The build endpoints state their input rules once

`POST /build/publish` opened with three guard clauses and `POST /build/upload`
with eight, each one a `return render json: { error: ... }` — name present,
version present, name format, version format, file present, filename prefix,
size, digest. That is a validation layer written by hand, in a place where it
cannot be unit tested: exercising it needs a request.

`PublishInputDto` was already there and was a bare `Struct` with no rules at
all, so the controller carried them. It is an `ActiveModel::Model` now, with
the presence and platform-inclusion validations on it, and `UploadInputDto`
joins it with the name and version formats and the `<name>-<version>` filename
convention. Each controller reads:

    return render json: { error: input.error_message }, status: :bad_request unless input.valid?

Size and digest keep their own explicit checks, because they are not the same
answer: 413 tells a caller to stop, 422 tells it to retry a truncated upload,
and a single error bag cannot say which. Every status code the endpoints
answered before, they answer now — build_publish_controller_spec and
build_uploads_controller_spec pin all of them, unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/dtos/warp_engine/publish_input_dto.rb

@@ -1,7 +1,14 @@
 module WarpEngine
-  PublishInputDto = Struct.new(:platform, :name, :version, keyword_init: true) do
-    def initialize(platform:, name:, version: nil)
-      super
-    end
+  class PublishInputDto
+    include ActiveModel::Model
+
+    attr_accessor :platform, :name, :version
+
+    validates :name, presence: true
+    validates :version, presence: true
+    validates :platform, presence: true
+    validates :platform, inclusion: { in: WarpEngine::Platform::NAMES }, allow_blank: true
+
+    def error_message = errors.full_messages.to_sentence
   end
 end

app/dtos/warp_engine/upload_input_dto.rb

@@ -0,0 +1,37 @@
+module WarpEngine
+  class UploadInputDto
+    include ActiveModel::Model
+
+    NAME_FORMAT = /\A[A-Za-z0-9._-]+\z/
+
+    attr_accessor :name, :version, :file, :sha256
+
+    validates :name, format: { with: NAME_FORMAT }
+    validates :version, format: { with: NAME_FORMAT }
+    validate :file_present
+    validate :filename_prefixed
+
+    def filename
+      return nil unless file.respond_to?(:original_filename)
+
+      File.basename(file.original_filename.to_s)
+    end
+
+    def error_message = errors.full_messages.to_sentence
+
+    private
+
+    def file_present
+      return if file.respond_to?(:original_filename)
+
+      errors.add(:file, "not provided")
+    end
+
+    def filename_prefixed
+      return if filename.nil? || errors.include?(:name) || errors.include?(:version)
+      return if filename.start_with?("#{name}-#{version}.", "#{name}-#{version}-")
+
+      errors.add(:file, "name must be prefixed with #{name}-#{version}")
+    end
+  end
+end