fix(delegate): inherit parent api_key when delegation.base_url set without delegation.api_key

[teknium1]

Salvage of #15853 by @briandevans onto current main.

Summary

When delegation.base_url is configured without delegation.api_key, the child agent was falling back to OPENAI_API_KEY from the env instead of inheriting the parent's resolved key. Parents running on non-OpenAI providers (Anthropic, Kimi, MiniMax, DashScope) saw their children fail auth because the inherited key was the wrong one.

Return api_key=None from _resolve_delegation_credentials when not configured, so _build_child_agent uses effective_api_key = override_api_key or parent_api_key — the parent's key, with explicit override still respected.

Conflict resolution during salvage

Main's docstring was updated for the related provider-resolution path since the PR opened. Merged docstrings so both code paths are documented. Updated two existing tests that locked in the old OPENAI_API_KEY-fallback behavior to expect the new inheritance semantics.

Changes

• tools/delegate_tool.py: return api_key=None for parent inheritance, update docstring (+6/-9)
• tests/tools/test_delegate.py: two existing tests renamed/rewritten for the new behavior

Validation

scripts/run_tests.sh tests/tools/test_delegate.py -k direct_endpoint -> 4 passed

Original PR: #15853
Fixes: #15810