Add gcovr.cfg to fix CI coverage merge errors (#1635)

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.

Author: baylesj

gcovr.cfg

@@ -0,0 +1,22 @@
+# Newer versions of gcovr have strict function merging by default, which
+# can cause issues with header-only functions or macros (like in jsontest.h).
+# 'separate' mode keeps them distinct, fixing the GcovrMergeAssertionError.
+merge-mode-functions = separate
+
+# --- Filtering ---
+# Only include the library sources in the coverage report.
+# This ensures coverage stats reflect the library quality and ignores test code.
+filter = src/lib_json/
+filter = include/json/
+
+# Exclude the build directory to avoid processing generated files
+exclude-directories = build
+
+# --- Noise Reduction ---
+# Ignore branches that are generated by the compiler (e.g., exception handling)
+# This drastically reduces "false positives" for missing branch coverage.
+exclude-throw-branches = yes
+
+# --- CI Visibility ---
+# Print a small summary table to the console logs.
+print-summary = yes