ci: bump codecov-action to v6 to fix signature verification

codecov-action@v5 verifies the codecov CLI against Codecov's old Keybase
key, which they bricked after migrating accounts. The upload step now
fails with "Could not verify signature" and, with fail_ci_if_error
true, hard-fails the Coverage job. v6 (= 6.0.2) ships the updated key.

Ref: codecov/codecov-action#1956

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: timtreis

.github/workflows/test.yaml

@@ -94,7 +94,7 @@ jobs:
           path: tests/figures/*
       - name: Upload coverage
         if: matrix.env.name == 'hatch-test.py3.14-stable'
-        uses: codecov/codecov-action@v5
+        uses: codecov/codecov-action@v6
         with:
           fail_ci_if_error: true
           use_oidc: true