fix(delegate): sync self.base_url with client_kwargs after credential resolution

[Tranquil-Flow]

Summary

Fixes #6825

When delegation.base_url routes subagents to a different endpoint, the correct URL is passed through _resolve_delegation_credentials() and _build_child_agent() into AIAgent.__init__(), but self.base_url can fall out of sync with client_kwargs["base_url"] — the value the OpenAI client actually uses.

This causes billing_base_url in session records to show the parent's endpoint while actual API calls go to the correct delegation target.

Root cause

At run_agent.py:852, self.api_key is synced from client_kwargs after credential resolution, but self.base_url (set earlier at line 584) is never updated to match. When the else branch at line 797 resolves credentials via resolve_provider_client(), the resolved base_url goes into client_kwargs but self.base_url stays stale.

Fix

One-line addition after line 852 — self.base_url = client_kwargs.get("base_url", self.base_url) — mirrors the existing self.api_key sync pattern. This also ensures the _primary_runtime snapshot (line 1241) captures the correct base_url for fallback recovery.

Test plan

• Existing delegation test suite passes (104 tests in test_delegate.py, test_delegate_toolset_scope.py, test_agent_loop.py)
• Verify with local dual-endpoint setup: parent on :8001, delegation target on :8000 — child session billing_base_url should show :8000

Platform

Tested on macOS (Darwin 24.6.0), Python 3.14.2

[teknium1]

Merged via PR #7360. Cherry-picked with authorship preserved. Textbook one-liner — thanks!