Introduce `RlmChildClient` — a dyn-compatible `#[async_trait]` wrapper around
the single create_message operation — so tests can inject a `MockRlmClient`
without a live API key. This replaces the direct `Arc<DeepSeekClient>` field
with `Arc<dyn RlmChildClient>`, wired transparently via `RlmQueryTool::new`.
Concurrency regression test (`rlm_parallel_fanout_overlaps_not_serialized`):
fires N=4 children each sleeping 50 ms through `join_all`. Asserts total
elapsed < 4×50 ms (serial bound) and that all start timestamps cluster within
<50 ms of each other. First run: total_elapsed=54 ms, start_spread=141 µs —
fan-out was already correct; no serialization fix needed.
UI wiring tests (`rlm_query_tool_cell_wired_with_prompts_on_start` etc.) verify
that `handle_tool_call_started` with `rlm_query` populates `GenericToolCell.prompts`
from the `prompts` (array) and `prompt` (singular) input shapes, and that
non-fan-out tools leave `prompts: None`.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
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