bacb90fe20
Completes the #90953 salvage on post-#98237 main: - New _merge_request_overrides helper defines the precedence contract: explicit delegation.request_overrides merges OVER runtime/parent-derived overrides — explicit top-level keys win; extra_body is deep-merged one level so runtime extra_body keys survive unless redefined. Inputs are copy.deepcopy'd so transport-side mutation can't leak into config or the provider runtime cache. - Direct base_url branch: explicit key now merges over the #98237 provider-alongside-base_url runtime overrides instead of being a separate return shape; max_output_tokens preserved. - Named-provider branch and parent-inherit branch now honor the key too, so delegation.request_overrides never silently no-ops. - _build_child_agent honors override_request_overrides whenever set (previously only when override_provider was set), enabling the inherit branch's merged value to reach the child. - DEFAULT_CONFIG: delegation.request_overrides entry with comment. - Tests: expanded tests/tools/test_delegate_request_overrides.py — deep-copy proofs, explicit-over-runtime precedence on the provider-alongside-base_url path, named-provider branch, inherit branch, and merge-helper unit tests. - Docs: configuration.md delegation section + features/delegation.md document the key, precedence, and example YAML (OpenRouter extra_body.provider.sort).