Problem

_spawn_background_review() creates a background review AIAgent without passing base_url or api_key. When the main agent uses a custom or fallback provider (e.g. OpenRouter, Cerebras, SiliconFlow), the review agent cannot:

1. Look up per-model context overrides from custom_providers (which requires base_url to match)
2. Query custom endpoint /models APIs for context length detection

This causes the review agent to fall through to live API detection, which may return the model's real context window and trigger the MINIMUM_CONTEXT_LENGTH (64K) guard — silently killing the review thread.

Fix

Pass base_url and api_key through to the review AIAgent constructor so it inherits the same provider configuration as the main agent.

Testing

• python3 -m py_compile run_agent.py — passes
• python3 -m pytest tests/run_agent/test_background_review_summary.py -v — 8/8 passed
• Verified the fix is present via inspect.getsource(AIAgent._spawn_background_review)

Closes #15882