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

Assisted contribution

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

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[devantler]

CLA is signed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.57%. Comparing base (64fb7f6) to head (dac265e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4560   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files         197      197           
  Lines       18291    18291           
=======================================
  Hits        18030    18030           
  Misses        261      261           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

According to the CLA checker:

The following contributors were found for this pull request:

   ✅ https://github.com/google/go-github/commit/dac265e8c161e10b58740ed4a426eccf1a8cecee Author: @devantler <niko********amm​@icloud.com>
   ❌ https://github.com/google/go-github/commit/dac265e8c161e10b58740ed4a426eccf1a8cecee Co-Author: @Copilot <223556219+Copilot​@users.noreply.github.com>

OK, so to resolve this problem, you are either going to need to recreate the commits locally so that ONLY YOU performed all the commits, and then FORCE-PUSH to this PR to eliminate the Copilot commits, or you are going to have to close this PR and open a brand new one where only you are the committer.

Sorry for the hassle, but we can't move forward with this PR until the CLA issue is resolved, per Google Open Source rules.

[devantler]

Replaced by #4561 with a newly created sole-author commit, as requested, so the CLA check can evaluate only the signed contributor. No force-push was used.