Fix coverage merge step: correct artifact paths and error on missing files

upload-artifact@v5 preserves directory structure, so downloaded artifacts
land at e.g. minitest-coverage/coverage/coverage.xml, not
minitest-coverage/coverage.xml.

Also change the rescue from StandardError (which silently swallowed missing
file errors) to only catch REXML::ParseException, and add an explicit abort
for missing files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nbudin

.github/workflows/ci.yml

@@ -326,7 +326,7 @@ jobs:
       - name: Check out repository
         uses: actions/checkout@v5
       - name: Merge coverage reports
-        run: ruby scripts/merge_coverage.rb merged-coverage.xml minitest-coverage/coverage.xml vitest-coverage/cobertura-coverage.xml minitest-system-coverage/coverage.xml
+        run: ruby scripts/merge_coverage.rb merged-coverage.xml minitest-coverage/coverage/coverage.xml vitest-coverage/coverage/cobertura-coverage.xml minitest-system-coverage/coverage/coverage.xml
       - name: Generate Coverage Report
         uses: clearlyip/code-coverage-report-action@v6
         id: code_coverage_report_action

scripts/merge_coverage.rb

@@ -18,6 +18,8 @@
 file_lines = Hash.new { |h, k| h[k] = Hash.new(0) }
 
 input_paths.each do |path|
+  abort "Error: coverage file not found: #{path}" unless File.exist?(path)
+
   doc = REXML::Document.new(File.read(path))
   doc
     .elements
@@ -27,7 +29,7 @@
         .elements
         .each("lines/line") { |line| file_lines[fname][line.attributes["number"].to_i] += line.attributes["hits"].to_i }
     end
-rescue StandardError => e
+rescue REXML::ParseException => e
   warn "Warning: could not parse #{path}: #{e}"
 end