executor buffer size should be customisable in no_std

Correct, users currently don't have a way to customise this. To be honest, I don't like that it isn't a dynamic allocation like it is on `std` with `async-executor`. I'm happy to revert this change so that _users_ don't see any difference, and then we follow-up in 0.17 with a larger refactor/re-write of `bevy_tasks` (hopefully including Forte by then).

_Originally posted by @bushrat011899 in https://github.com/bevyengine/bevy/pull/18506#discussion_r2009326484_

Start by tracing the no_std executor buffer handling in bevy_tasks and comparing it with async-executor's std behavior. Confirm how Forte relates to the proposed follow-up, then define a user-configurable buffer size and verify that no_std users can change it without altering the default behavior.

Written by the indexing model from the issue text.