Fix parent_tool_call foreign key inference

[Gujiassh]

Summary

• derive parent_tool_call from the tool-call class by default, but honor custom namespaced association prefixes when tool_calls_foreign_key is explicitly provided
• keep the fix scoped to the new acts_as API so it aligns with the already-correct legacy behavior without touching generators or docs
• add focused namespaced ActiveRecord regressions covering both the reflected foreign key and real parent_tool_call persistence with a custom tool_calls association name

Validation

• bundle exec rspec spec/ruby_llm/active_record/acts_as_spec.rb --example 'uses the tool_call_class-derived foreign key for parent_tool_call' --example 'persists parent_tool_call with a custom tool_calls association name'
• bundle exec rspec spec/ruby_llm/active_record/acts_as_spec.rb
• bundle exec rubocop lib/ruby_llm/active_record/acts_as.rb spec/ruby_llm/active_record/acts_as_spec.rb