yaml/compiler: audio::mixer num_inputs auto-injection silently no-ops on non-object params

[staging-devin-ai-integration]

Found while writing tests for crates/api/src/yaml/compiler.rs in PR #464 (Phase 3 coverage — Stream A).

Behavior

The `audio::mixer` `num_inputs` auto-injection in `compile_dag` only takes effect when the user-provided `params` is a JSON object. When `params` is present but not an object (e.g. a scalar, an array, a string), the injection silently no-ops and the compiled `audio::mixer` node ends up with no `num_inputs` set.

PR #464 pins this with the test `compile_dag_oneshot_audio_mixer_skips_inject_when_params_is_non_object`.

Why this matters

Pipelines that accidentally pass a non-object `params` for `audio::mixer` will compile silently but then fail at engine init or produce wrong behavior at runtime, with no compile-time signal pointing back to the user input.

Suggested resolution

In `compile_dag`, when a node's `kind` is `audio::mixer` and `params` is present but not an object, return a `Result::Err` with a clear message (e.g. `"audio::mixer params must be an object, got "`). This matches the existing error-on-bad-needs pattern in the same function.

Tracking

• Pinning test in PR test(api): cover yaml compiler, client_lint, and WS protocol round-trips #464: `compile_dag_oneshot_audio_mixer_skips_inject_when_params_is_non_object` in `crates/api/src/yaml/compiler.rs`.
• Filed by parent session https://staging.itsdev.in/sessions/a088e863cbbd473b9be4a57529fc674b at the end of the Phase 3 coverage initiative.