ci: use codecov/test-results-action for test analytics upload

The unified codecov/codecov-action@v6 with 'report_type: test_results'
doesn't actually invoke the right Codecov CLI subcommand — its
codecov.sh hardcodes CC_RUN_CMD=upload-coverage, so test-results
files were uploaded as coverage and never showed up in the Tests
tab (just '0 tests, no test results found').

Switch to the dedicated codecov/test-results-action@v1.2.1, which
explicitly runs 'do-upload --report-type test_results'.  Yes, this
action carries a 'deprecate' marker, but it's the only path that
works today — the Codecov docs themselves still recommend it, and
the deprecation is forward-looking for whenever codecov-action v6+
properly dispatches on report_type.

Author: glima

.github/workflows/ci.yml

@@ -49,10 +49,9 @@ jobs:
 
       - name: Upload test results to Codecov
         if: always()
-        uses: codecov/codecov-action@57e3a136b779b570ffcdbf80b3bdc90e7fab3de2 # v6.0.0
+        uses: codecov/test-results-action@6ba3fdeec616fb91fd6a389b788a2366835a0fa2 # v1.2.1
         with:
           files: target/nextest/applet/junit.xml,target/nextest/standalone/junit.xml
-          report_type: test_results
           token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: false