feat: support libuv finalization before task_manager

[algebraic-dev]

This PR fixes the finalization of the LibUV thread.

The finalization of the libuv thread occours in object.cpp inside the lean_finalize_task_manager immediately before the g_task_manager being deleted. We need that because we need this exact order and the finalize function runs after the manager is deleted.

extern "C" LEAN_EXPORT void lean_finalize_task_manager() {
    if (g_task_manager) {
        finalize_libuv();
        delete g_task_manager;
        g_task_manager = nullptr;
    }
}

Now event_loop_lock returns a boolean that says if the loop is closing or not and it's used like that:

if (!event_loop_lock(&global_ev)) {
    lean_dec(promise);
    free(resolver);
    return lean_uv_loop_unavailable_error();
}

We have two cond variables, one for waiting for new requests within the libuv loop and other one to the finalization process, so some finalizers can wait the finalization of the loop to close and free resources.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 0758b1d2e33c65ccea578abb0c668aab1f811608. You can force Mathlib CI using the force-mathlib-ci label. (2026-06-28 00:24:45)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 84b251f7390c20a0a00a221050ab4a6e6c46a191. You can force Mathlib CI using the force-mathlib-ci label. (2026-06-28 15:42:25)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 28b99ec01f6275235fedecbac8e382cac682bf80. You can force Mathlib CI using the force-mathlib-ci label. (2026-07-01 03:11:15)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 84b251f7390c20a0a00a221050ab4a6e6c46a191. You can force reference manual CI using the force-manual-ci label. (2026-06-28 00:24:46)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 4c59d1cde23b6f8143767d92ffdf4dc952300207 --onto 28b99ec01f6275235fedecbac8e382cac682bf80. You can force reference manual CI using the force-manual-ci label. (2026-06-30 19:33:50)