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

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

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

Keep self.base_url in sync with client_kwargs after the credential
resolution block, matching the existing pattern for self.api_key.

Fixes #6825

run_agent.py

@@ -947,6 +947,7 @@ class AIAgent:
                     client_kwargs["default_headers"] = headers
 
             self.api_key = client_kwargs.get("api_key", "")
+            self.base_url = client_kwargs.get("base_url", self.base_url)
             try:
                 self.client = self._create_openai_client(client_kwargs, reason="agent_init", shared=True)
                 if not self.quiet_mode: