Add request coalescing to `Runnable`

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What this task asks for

Add `Runnable.with_coalesce()` so concurrent identical inputs share one execution across sync, async, streaming, and batch APIs. Add `Runnable.with_coalesce()` so concurrent calls with the same input share one execution. Apply coalescing across synchronous and asynchronous invoke, stream, batch, and batch-as-completed methods. Build keys from input values alone, independent of configuration, keyword arguments, or dictionary key ordering. Replay complete streams to joiners, preserve batch ordering, and fire callbacks for joined callers. Provide synchronous and asynchronous backend registration, joining, completion, activity, and statistics operations. Keep the in-memory backend thread-safe and let clear cancel waiters and reset statistics.