Add gcovr.cfg to fix CI coverage merge errors

[baylesj]

Configures gcovr to use 'merge-mode-functions = separate', resolving the GcovrMergeAssertionError caused by strict merging of header-only functions in jsontest.h.

Also adds source filtering to focus reports on src/lib_json/ and include/json/, and excludes throw branches to reduce false positives.

[baylesj]

Of course this is impacted by a current outage (https://www.githubstatus.com/).

Update - We continue to investigate failures impacting GitHub Actions hosted-runner jobs.
We have identified the root cause and are working with our upstream provider to mitigate.
This is also impacting GitHub features that rely on GitHub Actions (for example, Copilot Coding Agent and Dependabot).
Feb 02, 2026 - 21:13 UTC