Fix Zeitwerk eager loading for railtie.rb (#486)

## Summary

Completes the fix from PR #59 to prevent crashes during eager loading in
non-Rails contexts.

## Problem

```
uninitialized constant RubyLLM::Rails (NameError)
  class Railtie < Rails::Railtie
```

Occurs when Zeitwerk eager loads (e.g., in gems that depend on ruby_llm
but don't use Rails).

## Root Cause

**PR #59** added conditional require: `require 'ruby_llm/railtie' if
defined?(Rails::Railtie)`

This is insufficient because Zeitwerk's eager loading happens *before*
that check runs. When Zeitwerk loads `railtie.rb`, the class definition
fails because `Rails::Railtie` doesn't exist.

## Solution

Two complementary fixes (both required per Zeitwerk documentation):

**1. Wrap class definition** in `if defined?(Rails::Railtie)`:
```ruby
if defined?(Rails::Railtie)
  module RubyLLM
    class Railtie < Rails::Railtie
      # ...
    end
  end
end
```

**2. Ignore from Zeitwerk**:
```ruby
loader.ignore("#{__dir__}/ruby_llm/railtie.rb")
```

## Evidence

- **Zeitwerk issue #143**: Maintainer confirms conditional loading
pattern
- **Zeitwerk docs**: Use `loader.ignore()` for "files not following
conventions"
- **Real-world example**: github/secure_headers uses same pattern

## Testing

**Before**: `Zeitwerk::Loader.eager_load_all` → ❌ NameError  
**After**: `Zeitwerk::Loader.eager_load_all` → ✅ Success

Thank you for reviewing!

Co-authored-by: Carmine Paolino <[email protected]>

Author: trevorturk

lib/ruby_llm.rb

@@ -29,6 +29,7 @@
 )
 loader.ignore("#{__dir__}/tasks")
 loader.ignore("#{__dir__}/generators")
+loader.ignore("#{__dir__}/ruby_llm/railtie.rb")
 loader.setup
 
 # A delightful Ruby interface to modern AI language models.

lib/ruby_llm/railtie.rb

@@ -1,33 +1,35 @@
 # frozen_string_literal: true
 
-module RubyLLM
-  # Rails integration for RubyLLM
-  class Railtie < Rails::Railtie
-    initializer 'ruby_llm.inflections' do
-      ActiveSupport::Inflector.inflections(:en) do |inflect|
-        inflect.acronym 'RubyLLM'
+if defined?(Rails::Railtie)
+  module RubyLLM
+    # Rails integration for RubyLLM
+    class Railtie < Rails::Railtie
+      initializer 'ruby_llm.inflections' do
+        ActiveSupport::Inflector.inflections(:en) do |inflect|
+          inflect.acronym 'RubyLLM'
+        end
       end
-    end
 
-    initializer 'ruby_llm.active_record' do
-      ActiveSupport.on_load :active_record do
-        if RubyLLM.config.use_new_acts_as
-          require 'ruby_llm/active_record/acts_as'
-          ::ActiveRecord::Base.include RubyLLM::ActiveRecord::ActsAs
-        else
-          require 'ruby_llm/active_record/acts_as_legacy'
-          ::ActiveRecord::Base.include RubyLLM::ActiveRecord::ActsAsLegacy
+      initializer 'ruby_llm.active_record' do
+        ActiveSupport.on_load :active_record do
+          if RubyLLM.config.use_new_acts_as
+            require 'ruby_llm/active_record/acts_as'
+            ::ActiveRecord::Base.include RubyLLM::ActiveRecord::ActsAs
+          else
+            require 'ruby_llm/active_record/acts_as_legacy'
+            ::ActiveRecord::Base.include RubyLLM::ActiveRecord::ActsAsLegacy
 
-          Rails.logger.warn(
-            "\n!!! RubyLLM's legacy acts_as API is deprecated and will be removed in RubyLLM 2.0.0. " \
-            "Please consult the migration guide at https://rubyllm.com/upgrading-to-1-7/\n"
-          )
+            Rails.logger.warn(
+              "\n!!! RubyLLM's legacy acts_as API is deprecated and will be removed in RubyLLM 2.0.0. " \
+              "Please consult the migration guide at https://rubyllm.com/upgrading-to-1-7/\n"
+            )
+          end
         end
       end
-    end
 
-    rake_tasks do
-      load 'tasks/ruby_llm.rake'
+      rake_tasks do
+        load 'tasks/ruby_llm.rake'
+      end
     end
   end
 end