input: fixed threaded input processor initialization

[edsiper]

This is a rebased PR of #10350 on top of master branch (trying to workaround some CI issues)

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[edsiper]

last CI issue is not in our side , github runner is exiting right away.

[drbugfinder-work]

Hi @leonardo-albertovich, @edsiper!

This PR implements the threaded input event-loop binding discussed in #9835. The direction is right, but in practice it appears insufficient and introduces a regression with timer/notification handling in threaded inputs.

After this change, timer/notification events in the per-thread loop can remain effectively always ready, causing a tight epoll_wait(0) spin and pushing CPU usage to ~100%. See #10529
Because of this hot-loop behavior we currently can’t reliably validate whether the original problem in #9835 is actually fixed under load.

@leonardo-albertovich could you provide a follow-up that completes the fix so timer/notification events in threaded inputs don’t trigger the non-stop epoll_wait(0) loop? Once available, we’ll retest and confirm #9835 without reintroducing the 100% CPU issue from #10529.

cc @lecaros