Fixed basic_json::diff and basic_json::patch not compiling with custom string type (#4134)

[tomalakgeretkal]

Note that this has yet to be amalgamated (waiting for #4083).

[tomalakgeretkal]

The template version broke SFINAE-driven feature-detection in detail/string_concat.hpp.

[tomalakgeretkal]

Amalgamated.

[nlohmann]

Please update from the develop branch as it has a working CI now.

[coveralls]

coverage: 100.0%. remained the same when pulling 96129bd on tomalakgeretkal:custom-string-fix into edffad0 on nlohmann:develop.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @tomalakgeretkal
Please read and follow the Contribution Guidelines.

[tomalakgeretkal]

Looks amalgamated here :( check-amalgamate works for me!

[gregmarr]

You may be using a different version of astyle that produces different results. The check is using 3.1. We've seen that later versions produce different results.

[github-actions]

This pull request has been marked as stale because it has had no activity for 30 days. While we won’t close it automatically, we encourage you to update or comment if it is still relevant. Keeping pull requests active and up-to-date helps us review and merge changes more efficiently. Thank you for your contributions!

[nlohmann]

Thank you very much for this PR, @tomalakgeretkal, and for reporting #4134. The fix landed in #4536, which lets patch and diff work with arbitrary string types, and it is already on develop. I'm closing this PR as superseded. Thanks again!

_{This comment was written by Claude Code on behalf of @nlohmann.}