## Description

This PR fixes issue #397 where the upgrade generator produces invalid
table names for namespaced models.

## Problem

When using custom model names with namespaces (like `Assistant::Chat`,
`Assistant::Message`, etc.) and running the upgrade generator:

```bash
rails g ruby_llm:upgrade_to_v1_7 chat:Assistant::Chat message:Assistant::Message
```

The migration would generate invalid table names like `:assistant/chats`
instead of `:assistant_chats`.

## Solution

The issue was that the generator was using `.tableize` which produces
forward slashes for namespaced models. This has been fixed by using
`.underscore.pluralize.tr('/', '_')` which properly converts namespaced
model names to valid table names.

### Changes:
- Added `table_name_for` helper method that correctly handles namespaced
models
- Added helper methods for each model type (`chat_table_name`,
`message_table_name`, etc.)
- Updated migration template to use the new helper methods instead of
`.tableize`

## Testing

Tested with namespaced models to confirm:
- `Assistant::Chat` → `assistant_chats` ✓
- `Assistant::Message` → `assistant_messages` ✓
- `Assistant::ToolCall` → `assistant_tool_calls` ✓
- `Assistant::Model` → `assistant_models` ✓

Regular models continue to work as expected:
- `Chat` → `chats` ✓
- `Message` → `messages` ✓

Fixes #397

- Move each generator into its own subdirectory
- Update source paths to work from new locations
- Add table_name_for helper to handle namespaced models
- Fix chat_ui generator to properly handle namespaced model table names
- Place private methods in conventional location at end of class

…399)

Fixes
#397 (comment) and
continues work from #398

- Fix instance variable usage in migration template
- Fix generators when using namespaced models
- Simplify upgrade instructions to point to migration guide
- Add troubleshooting section for acts_as_model error from issue #400
- Add warning about acts_as_model error directly in generator output
- Create initializer if missing during upgrade
- Use consistent helper methods across install and upgrade generators

- Upgrade generator now automatically rewrites existing models' acts_as declarations
- Updates Chat, Message, and ToolCall models to include proper model associations
- Create shared GeneratorHelpers module to DRY up generator code
- Add automatic namespace module creation with table_name_prefix for namespaced models
- Fix template paths in upgrade generator to use source_paths correctly
- Update docs to highlight automatic acts_as updates as key feature
- Delete implementation detail tests that check source code strings