[WIP] AppEngine: Implement property-based tests for `cast_value` functions

[Hibe7]

Implement property-based tests for cast_value functions.

• [x] I have read CONTRIBUTING.md and CODE_OF_CONDUCT.md
• [x] I have added to CHANGELOG.md relevant changes and any other user facing change
• [x] I have added or ran the appropriate tests

What this PR does / why we need it:

• Implement property-based tests for cast_value functions.

Special notes for your reviewer:

• The review of this PR can proceed after this PR gets accepted.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 64.05%. Comparing base (c737fc5) to head (d50b08b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.2 #980 +/- ## =============================================== - Coverage 69.62% 64.05% -5.57% =============================================== Files 264 208 -56 Lines 7097 5659 -1438 =============================================== - Hits 4941 3625 -1316 + Misses 2156 2034 -122 ```

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