# 099 — Remove `test_handle`'s duplicate-of-`spawn` test stub

## Summary

`test_handle` is a `#[cfg(test)]` production-file function that exists
only to give tests access to the engine's internal `mpsc::Receiver` —
it duplicates `spawn()` almost line-for-line rather than exposing that
receiver through `spawn()`/`EngineHandle` itself. Per this repo's
`IDEAS.md` guidance: prefer changing a function's scope/visibility (or
the tests) over maintaining a parallel stub.

## Where

- `src/engine/handle.rs:113-128` — `#[cfg(test)] pub(crate) fn
  test_handle(channel_capacity: usize) -> (EngineHandle,
  mpsc::Receiver<EngineMessage>)`, duplicating `spawn()`
  (`src/engine/handle.rs:103-111`) except that `spawn()` discards the
  receiver after wiring the actor loop, while `test_handle` returns it
  raw.
- Re-exported at `src/engine/mod.rs:21`: `pub(crate) use
  handle::test_handle;`.
- Consumed only by tests in `src/engine/ingest.rs:228` (`use
  crate::engine::test_handle;`) and called at lines 248, 264, 281, 302
  inside `#[cfg(test)] mod tests`.

## Scope

- Remove `test_handle` and instead give tests access via `spawn()`
  itself — e.g. have `spawn()` take an optional channel-capacity
  override and always hand back the receiver alongside the handle (or
  the actor join handle), with production call sites (`main.rs`)
  simply ignoring what they don't need, rather than keeping a second
  constructor whose only difference is what it returns.
- Update the four call sites in `src/engine/ingest.rs`'s test module
  to use the adjusted `spawn()` signature.
- Out of scope: the other test-fixture helpers in `src/engine/tests.rs`
  (`metric_request`, `declare_test_param`, `ingest_log`, `logs_query`,
  `logs_query_sorted`, etc.) are ordinary test-data builders, not
  visibility workarounds — leave those alone. Also leave the
  `#[allow(dead_code)]` items (`src/engine/message.rs:11,29`,
  `src/api/mcp_server.rs:254`, `src/config/mod.rs:27`, and similar in
  `metric_primitive.rs`/`data_value.rs`/`timestamp.rs`/`index_key.rs`)
  untouched — they weren't confirmed to be test-visibility stubs and
  need individual inspection before deciding whether they're even
  related to this ticket's pattern.