feat: Support explicit custom property values

[devantler]

Summary

CustomProperty could not represent GitHub’s optional require_explicit_values setting, preventing clients such as the Terraform provider from preserving or managing it.

This adds the optional field, keeps omitted and explicit false values distinct, and covers response decoding plus batch and single-property request bodies.

Closes #4559
Supersedes #4560 to resolve contributor CLA attribution.

Assisted contribution

GitHub Copilot substantially assisted with the code, tests, and prose. I reviewed, understood, and tested every change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (72a2f27) to head (b8607ba).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4561   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files         197      197           
  Lines       18299    18299           
=======================================
  Hits        18038    18038           
  Misses        261      261           

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[devantler]

@Not-Dhananjay-Mishra and @gmlewis it should be ready for approval now then :-)

[devantler]

@Not-Dhananjay-Mishra and @gmlewis applied the second round of review file suggestions.

[gmlewis]

Excellent, thank you, @Not-Dhananjay-Mishra and @devantler!
Sorry that I didn't provide helpful feedback right up front to save you a good deal of time, and thank you, @Not-Dhananjay-Mishra for guiding us here.
LGTM.

@Not-Dhananjay-Mishra - has all your feedback been addressed?

[devantler]

Excellent, thank you, @Not-Dhananjay-Mishra and @devantler! Sorry that I didn't provide helpful feedback right up front to save you a good deal of time, and thank you, @Not-Dhananjay-Mishra for guiding us here. LGTM.

@Not-Dhananjay-Mishra - has all your feedback been addressed?

No worries, we all do this all the time 😃 I am happy we got it as we want it, as that is the important thing after all!

[devantler]

This PR will be reviewed after we have gotten dev support into gh-token-exchanger :-) I still need to look through the code a bit before then.

[Not-Dhananjay-Mishra]

LGTM 🙂

[gmlewis]

Thank you, @Not-Dhananjay-Mishra!
Merging.