Remove implicit forking in ZIO#onDone and ZIO#onDoneCause to align with onX methods

[asr2003]

Updated onDone and onDoneCause to execute callbacks synchronously on the main fiber to align behavior with other onX methods removing implicit forking( (i.e. NOT fork).

Closes #9191
/claim #9191

Important Note about Changes to onDone and onDoneCause Behavior

In this release, we have updated the behavior of onDone and onDoneCause to address concerns raised in Issue #9191. Previously, these methods operated asynchronously by forking their callback effects, which could lead to subtle bugs and race conditions. This behavior was unique among ZIO's onX methods and diverged from typical expectations.

Key Behavior Change: Synchronous Execution

Starting in this release, onDone and onDoneCause now execute success and failure callbacks synchronously within the calling fiber. This update aligns them with other onX methods (e.g., onExit), ensuring that the effect completes fully, including any specified callbacks, before moving on. This change provides greater predictability and reliability by removing the previously implicit asynchronous behavior.

Summary and Impact

This enhancement addresses the risk identified in Issue #9191, where the unexpected forking could cause surprising, hard-to-trace bugs. As a result, users may notice different callback behaviors following this upgrade. Where necessary, alternative patterns (such as explicit forking) can be employed to replicate prior behavior. We encourage users to review any code that relies on onDone or onDoneCause and verify that the updated synchronous behavior aligns with expected outcomes.

[jdegoes]

@asr2003 These need tests to guarantee synchronous behavior for each.

[asr2003]

@jdegoes Sure! I will add it

[kyri-petrou]

@asr2003 in addition to John's comment, can you please do the following:

1. Add scaladoc to both onDone and onDoneCause methods
2. Write a paragraph explaining the behaviour change of onDone which we will use in release notes. This should be similar to the "Important note about ZPool changes" found in the v2.1.10 notes

[kyri-petrou]

One last comment, I think the implementation should use onExit so that we're consistent across onX implementations

[asr2003]

@kyri-petrou @jdegoes Done with the changes!

[asr2003]

@kyri-petrou There has been type error if we use OnExit directly on OnDoneCause

error] /home/runner/work/zio/zio/core/shared/src/main/scala/zio/ZIO.scala:1074:12: type mismatch;
[error]  found   : zio.ZIO[R1,E,A]
[error]  required: zio.ZIO[R1,Nothing,Unit]
[error]     onExit {
[error]            ^

So I have updated for the same to resolve this type mismatch

[asr2003]

@kyri-petrou Done with the changes!