Context

RuntimeLayer in crates/zeph-core/src/runtime_layer.rs supports multiple layers and short-circuit via before_chat. Tests added in PR #2358 cover basic ordering, but the following are not yet tested:

Missing tests

1. Multi-layer before_tool → after_tool ordering (two layers, verify call sequence)
2. after_chat receiving the response from a short-circuited before_chat (i.e., when a layer returns Some(response), after_chat should receive that response)

Acceptance criteria

#[tokio::test] tests in runtime_layer.rs for both cases.

Discovered in PR #2358 tester review.