[Pipeline] Support async ops in fused subprocess stages

[mthrok]

fuse_subprocess_stages=True fuses runs of adjacent pipe stages that share the same isolating-pool (process/interpreter) executor instance into one nested Pipeline that runs inside a worker process, eliminating the per-stage IPC round-trip. Previously an async op could never be part of a fused run: PipeConfig rejected any executor on an async op, and fusion groups stages purely by executor-instance identity. An async op between two pool stages therefore split the fusable run in two, forcing the intermediate value to round-trip (and be pickled) through the main process.

This commit lets an async op join a fused run by tagging it with the same isolating-pool executor as its neighbours. The executor is never used to run the coroutine (an async op always runs on the event loop) — it is only the fusion-group key. When fused, the tag is stripped and the op runs on the worker's own event loop, exactly as fused path_variants async branches already do. When not fused (fusion off, or a lone async op), the tag is ignored and the op runs on the main loop as before. The execution engine is unchanged: each worker already rebuilds the sub-config with the normal build_pipeline, which runs a full event loop.

Details:

• PipeConfig.__post_init__ now allows an isolating-pool executor on an async op and rejects only non-isolating executors (e.g. a thread pool), which have no effect on an async op.
• convert_to_async ignores the executor for async ops instead of asserting it is None.
• _is_isolating_pool/_is_interpreter_pool moved from _fuse to _common/_convert so defs can reuse them without an import cycle.
• Fusion detection: an async op that now carries an executor flows through _fusable_pool_executor automatically. Inside a path_variants stage, _scan_variant_pool_executors ignores an async op's pool tag instead of treating it as an input-ordered pool-pipe — an async op's executor is only a fusion-group tag (it runs on the loop, not the pool), so its output_order="input" cannot be broken by pool parallelism and must not block fusing the same-pool stage. Only a sync input-ordered pool-pipe still blocks fusion. _stage_concurrency counts async ops as zero worker threads (they run on the loop, not the thread pool).
• run_pipeline_in_subprocess strips any executor tag left on an unfused async op before the op-agnostic executor-hoisting pass, so a tag never spawns an idle worker pool the op will not use.

A fused async op must be picklable, like any fused stage. This is documented on PipelineBuilder.pipe and in the parallelism guide.

[meta-codesync]

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