fix(agent-bus): let a host's loop hooks reach sub-agents

[zhanghanduo]

Summary

Found reviewing ApodexAI/MiroHarness#513, which replaces the product's own agent_bus/bus.py with a facade onto this one.

AgentBus called run_agent_loop with neither runtime_hooks nor parser. A host almost always wraps the engine with its own hooks, so the bus handed those to the agents the host launches itself and silently withheld them from every sub-agent it spawns. The asymmetry has no runtime symptom — the loop still runs, just with all-default no-op hooks.

What that costs the host that found it: per-tool timeout floors, the overflow spill-file recovery path (both the transform_result writer and body_has_spill_reference, so "go read the spill file" could never fire inside a sub-agent), sub-agent ExecutionScope entry (cost and traces attributed to the parent's role instead of the sub-agent's), session affinity, and the parser policy that answers a hallucinated tool name with the list of available tools.

Design note: seams, not a runner swap

The first attempt was a single configure_default_loop_runner that replaced run_agent_loop outright. That shadows the module-global name unit tests monkeypatch to keep a real loop out of a test — nine test modules in MiroHarness alone — so those tests would have run the real loop and hung rather than failed. Verified: the product suite wedged at 34%.

So this uses the engine's own runtime_hooks / parser seams instead. The bus keeps calling run_agent_loop by name; a host supplies resolvers for the two things it actually owns. Resolvers rather than values so a host sitting below the loop package in its own layer stack can import lazily (MiroHarness enforces that with a kernel-purity test).

test_a_monkeypatched_engine_still_keeps_a_loop_out_of_a_unit_test pins that property.

Also: Any parameters lost their schema type

The extraction commit added if t is Any: return {} to _schema_for_type, changing the pre-port fall-through to {"type": "string"}. An empty sub-schema reads as "anything goes", but strict function-schema validators — Gemini-compatible gateways among them — reject a property with no type, so a tool declared list[Any] started emitting items: {} and could 400 a whole fallback leg on schema validation alone. Restored, with a comment pointing a tool that means "object" at dict[str, Any].

Tests

• tests/test_agent_bus_host_loop_seams.py (6) — both seams reach both dispatch paths, including the deferred session drain; resolution is at call time; the patchable name is not shadowed.
• tests/test_tool_schema_types.py (6) — no emitted property, at any nesting depth, lacks a type.

Validation

• ruff check agent_core tests clean; pytest -q 921 passed.
• Consumer check with MiroHarness#513 pointed at this branch: 7148 passed, 72 skipped, 3 xfailed.
• Merges clean with fix(agent-bus): establish a dispatched session task, and cancel a round cleanly #17 (fix/session-spawn-yield) — different hunks of bus.py, no semantic overlap. Verified on a throwaway merge: AgentCore 923 passed, MiroHarness 7148 passed.