keep every pending registration per task instead of replacing it

[cicoyle]

The local tasks backend kept one completion registration per task (activity: instance ID + task ID, workflow: instance ID) and stored a new one with a plain sync.Map.Store. A second registration for the same task replaced the first. Complete and cancel only reached the newer one, and the older one's deregister was a no-op, so the older execution stayed pending until the worker context ended.

In Dapr this happens when a workflow instance is recreated under the same ID while an activity of the previous run is still executing on the same sidecar.

Stress tests showed sidecars holding about 90 such executions each (about 250 goroutines and 30 MB), released only at shutdown with failed before receiving activity result. With WithMaxParallelism, each stranded execution also held a worker slot forever.

[Copilot]

🟡 Changes recommended

Tokenless completions can settle overlapping executions with another run’s result.

1 open finding

• Tokenless completions incorrectly settle multiple executions sharing a key · New

What changed in this PR

Keeps all pending registrations for each local task so overlapping executions are not stranded by newer registrations.

Changes:

• Replaces single-registration storage with a synchronized callback registry.
• Adds delivery, deregistration, concurrency, and executor regression tests.
• Updates the callback contract and enables backend race tests in CI.

File	Description
backend/​local/​task.go	Retains and delivers to every same-key registration.
backend/​local/​task_test.go	Tests multiple registrations and concurrent use.
backend/​local/​executor_test.go	Tests overlapping executions with distinct completion tokens.
backend/​backend.go	Documents delivery to all pending callbacks.
.github/​workflows/​pr-validation.yml	Runs backend tests with race detection.

🧠 Review effort: Balanced

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