[Variant] List and object builders have no effect until finalized (#7865)

# Which issue does this PR close?

- Closes #7863
- Closes #7798

# Rationale for this change

Reviews and testing on #7843
exposed the fact that creating a variant list or object builder has side
effects that leave the parent in an inconsistent/invalid state if the
child builder is never finalized. Rework the finalization logic to be
more direct so that child builders have no effect on their parents
before their `finish` method is called.

# What changes are included in this PR?

* Define a new `ParentState` enum that tracks the necessary information
for a child to fully finalize its parent.
* Remove the `pending` machinery from builders

# Are these changes tested?

Existing unit tests mostly cover this change. 

Added new tests to verify that failing to call `finish` leaves the
parent unmodified.

# Are there any user-facing changes?

No.

Author: scovich