fix: preserve runtime in background review clones

[guantik-gm]

Summary

• preserve base_url, api_key, and api_mode when _spawn_background_review() forks a review agent
• prevent named custom providers resolved to provider='custom' from degrading into a bare custom/main review clone
• add regression coverage for memory, skill, and combined background-review modes

Problem

Gateway/main sessions using named custom providers are runtime-resolved to provider='custom' plus explicit runtime credentials. Background review cloned only model + provider, so the fork lost endpoint state and failed with:

• resolve_provider_client: custom/main requested but no endpoint credentials found
• Auxiliary background review failed: No LLM provider configured

Test Plan

• pytest tests/run_agent/test_background_review_runtime_inheritance.py tests/run_agent/test_background_review_summary.py -q
• minimal runtime repro: clone with only model + provider fails; clone with inherited runtime fields succeeds

[alt-glitch]

Related to #7491, #2581, and #15884 — all address the same background review runtime inheritance issue. This PR adds additional regression test coverage.

[teknium1]

Thanks for the regression-focused fix. This automated hermes-sweeper review found that the requested behavior is already on main.

• run_agent.py:1155-1163 exposes the parent session's live base_url, api_key, and api_mode through _current_main_runtime().
• agent/background_review.py:55-64 resolves that live runtime, and agent/background_review.py:683-692 forwards provider, api_mode, base_url, api_key, and credential_pool to the review AIAgent.
• The linked runtime-inheritance discussion was resolved by merged PR fix(run_agent): background review fork inherits parent's live runtime #16099 (e3901d5b257d5ac3f58420c3fb55aaa536fc56ac); the current refactored implementation is included in v2026.7.1.

Closing as implemented on main.