This fleshes out the remainder of reviewboard.diffviewer.commit_utils with typing and documentation. The validation payloads are now fully typed, which is only useful internally as this is considered opaque outside this module. There is one fix with equality checking for CommitHistoryDiffEntry. It assumed the other type, and would crash if given a different type. We now check for this. This is mostly the same as Christian's change at /r/14416. The changes I've made: - Moved `SerializedCommitHistoryDiffEntryType` so it's defined before it's used. - Fixed a typo in the `DiffCommitsValidationInfo` docs. - Fixed up some Args/Returns types to list the specific TypedDict/TypeAlias names instead of just `str` or `dict`. Testing Done: Ran unit tests.