Prevent leaking parent provider config by discarding inherited options for different services

[TheRealNeil]

Prevent parent provider config leaking when child agent switches service

Fixes #304

Summary

• Discard all inherited options when the child agent's provider service differs from the parent's, instead of only stripping :service and :api_version

Root cause

When a child agent overrides generate_with with a different provider (e.g. parent uses Ollama, child uses Azure), the merge in generate_with only removed :service and :api_version from inherited options. All other parent options (:host, :api_key, :model, etc.) survived and — because the merge order is global_options.merge(inherited_options) — overrode the child's YAML config. This caused requests to hit the wrong endpoint with the wrong credentials.

Fix

When global_options[:service] != inherited_options[:service], replace inherited_options.extract!(:service, :api_version) with inherited_options = {}. This gives the child a clean slate from its own YAML config when switching providers, while preserving same-provider inheritance (e.g. a child that only tweaks temperature).

Test plan

• Child agent with a different generate_with provider uses its own YAML config (host, api_key, model)
• Child agent with the same provider still inherits parent options correctly
• Agent-level overrides (e.g. generate_with :azure, model: 'gpt-4.1') still take precedence