Flaky asset processing test due to multiple processing tasks for same path.

[andriyDev]

CI: https://github.com/bevyengine/bevy/actions/runs/19775398295/job/56667076447

The failing test is processor::tests::only_reprocesses_wrong_hash_on_startup, with the failure:

assertion `left == right` failed
  left: 3
 right: 2

Based on my debugging, I believe the situation is that the dep_changed asset queues the initial processing task, but then source_changed completes its task and enqueues dep_changed to be reprocessed and now we have two tasks for the same asset and somehow that seems to block (see below).

Repro

while true; do RUST_LOG=bevy_asset=trace cargo t -p bevy_asset --features=multi_threaded wrong_hash --tests; if [ $? -ne 0 ]; then break; fi; done

Note: We don't use the log plugin in bevy_asset tests, so I needed to add bevy_log as a dev dependency:

[dev-dependencies]
bevy_log = { path = "../bevy_log", version = "0.18.0-dev" }

And add the bevy_log::LogPlugin::default() to create_app_with_asset_processor.

Doing all this, I actually get a "ran out of loops" error - this means that the code actually blocks. This might mean there are two bugs, but I think they are related. Unfortunately, adding enough traces seems to prevent the blocking, so I am not sure where it is blocking.