Re-add minimal Rails shim for the tidewave.ai frontend

The frontend (and the project_eval connectivity probe) evaluates Ruby
that references `Rails.env`, `Rails.application.config.*`, etc. to
confirm the host is alive. Under Sinatra / plain Rack / standalone
processes there is no Rails constant, so the probe raises NameError
and the UI reports "Could not connect to your app".

Add Tidewave::RailsShim, auto-installed from Tidewave::Middleware#initialize
when no real Rails is defined:

- StringInquirer (subclass of String) answers development?/test?/production?
  so frontend env predicates work.
- DeepStub returns self for any method call so chains like
  Rails.application.config.something resolve harmlessly instead of raising.
- FakeRails exposes env, root, logger, application, version backed by
  Tidewave.config where possible.

install! is idempotent and a no-op when real Rails is already loaded,
so this is safe to include in Rails hosts too.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tomazzlender

lib/tidewave/middleware.rb

@@ -8,6 +8,7 @@
 require "logger"
 require "fileutils"
 require "tidewave"
+require_relative "rails_shim"
 require_relative "streamable_http_transport"
 
 # Tools register themselves as descendants of Tidewave::Tools::Base, so we
@@ -29,6 +30,7 @@ class Tidewave::Middleware
   INVALID_ORIGIN = "For security reasons, Tidewave does not accept requests with an origin header for this endpoint.".freeze
 
   def initialize(app, config = nil)
+    Tidewave::RailsShim.install!
     @config = config || Tidewave.config
     @allow_remote_access = @config.allow_remote_access
     @client_url = @config.client_url

lib/tidewave/rails_shim.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+# Minimal Rails-API stub for the tidewave.ai frontend and the Tidewave
+# connectivity probe, which evaluate things like `Rails.env` or
+# `Rails.application.config.something` to confirm the app is alive.
+#
+# This is NOT Rails. It answers just enough of the surface so probes
+# don't raise NameError when running under Sinatra, plain Rack, workers,
+# or standalone scripts.
+#
+# Auto-installed by Tidewave::Middleware (and Tidewave::Server) when no
+# real Rails constant is defined. Hosts that already have Rails get the
+# real one and skip this entirely.
+module Tidewave
+  module RailsShim
+    # String-like that responds to development?/test?/production? etc.,
+    # matching ActiveSupport::StringInquirer behavior closely enough for
+    # the frontend's environment checks.
+    class StringInquirer < String
+      def method_missing(name, *args)
+        name = name.to_s
+        return self == name.chomp("?") if name.end_with?("?")
+
+        super
+      end
+
+      def respond_to_missing?(name, include_private = false)
+        name.to_s.end_with?("?") || super
+      end
+    end
+
+    # Returns itself for any method call so chained probes like
+    # Rails.application.config.something resolve harmlessly to an
+    # inspectable sentinel rather than raising NoMethodError.
+    class DeepStub
+      def method_missing(_name, *_args, &_blk) = self
+      def respond_to_missing?(_name, _ = false) = true
+      def to_s = ""
+      def inspect = "#<Rails (Tidewave shim)>"
+    end
+
+    # The faux Rails module installed at the top level. Mirrors the
+    # handful of methods the tidewave.ai frontend touches.
+    module FakeRails
+      module_function
+
+      def env
+        @env ||= StringInquirer.new(
+          Tidewave.config.environment || ENV.fetch("RACK_ENV", "development")
+        )
+      end
+
+      def root
+        @root ||= Tidewave.config.root || Pathname.new(Dir.pwd)
+      end
+
+      def logger
+        @logger ||= Tidewave.config.resolved_logger if Tidewave.config.respond_to?(:resolved_logger)
+        @logger ||= Logger.new($stdout)
+      end
+
+      def application
+        @application ||= DeepStub.new
+      end
+
+      def version = "0.0.0-tidewave-shim"
+    end
+
+    # Define ::Rails as the FakeRails module if no real Rails is loaded.
+    # Idempotent: a second call (e.g., after a real Rails appears later)
+    # is a no-op.
+    def self.install!
+      return if defined?(::Rails)
+
+      require "logger"
+      require "pathname"
+      Object.const_set(:Rails, FakeRails)
+    end
+  end
+end