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

[Copilot]

Pull request overview

Fixes a configuration inheritance bug where a child agent switching to a different provider service could unintentionally inherit (and override) its own provider settings with parent provider options.

Changes:

• When generate_with detects a provider :service change between parent and child, it now discards all inherited options (instead of only stripping :service and :api_version).
• Adds clarifying inline comments explaining the rationale (prevent leaking host/api_key/model, etc.).

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[Copilot]

This changes provider option inheritance behavior in a subtle way (clearing all inherited options when the service changes), but there’s no automated test coverage validating the new behavior. Please add tests that cover: (1) parent/child with different service does not inherit provider-specific keys like host/api_key/model, and (2) same-service inheritance still works, and (3) explicit generate_with overrides still win over YAML config/inheritance.