Reject agents.max_threads with multi_agent_v2

[jif-oai]

Why

multi_agent_v2 uses the v2 agent lifecycle, so accepting the legacy agents.max_threads limit alongside it creates conflicting configuration semantics. Config load should fail early with a clear error instead of allowing both knobs to be set.

What Changed

• During config load, detect when the effective multi_agent_v2 feature is enabled and agents.max_threads is explicitly set.
• Return an InvalidInput error: agents.max_threads cannot be set when multi_agent_v2 is enabled.

Verification

• cargo test -p codex-core multi_agent_v2_rejects_agents_max_threads passed locally with a temporary focused test for this behavior.
• cargo test -p codex-core was also run; the new focused path passed, but the crate suite has unrelated pre-existing failures in managed config/proxy/request-permissions tests.