fix: cover fixDiscovered formatDiagnostics call for codecov/patch

Add TestE2E_Fix_Discovered_UnfixableDiagnostic, which runs fix in
discovery mode with a file that has an unfixable MDS017 diagnostic.
After the fix pass, the diagnostic remains and formatDiagnostics is
called in fixDiscovered (line 855), closing the only uncovered patch
line from the previous commit.

https://claude.ai/code/session_018673HUFUK6ceA9YyxH3HKg

Author: claude

cmd/mdsmith/e2e_coverage_test.go

@@ -756,3 +756,21 @@ func TestE2E_Check_Stdin_Quiet(t *testing.T) {
 	assert.NotContains(t, stderr, "MDS006",
 		"expected no diagnostic output with --quiet stdin, got: %s", stderr)
 }
+
+// =============================================================
+// fixDiscovered with unfixable diagnostics (non-quiet)
+// =============================================================
+
+func TestE2E_Fix_Discovered_UnfixableDiagnostic(t *testing.T) {
+	dir := t.TempDir()
+	isolateDir(t, dir)
+	// trailing-punctuation in heading is unfixable; trailing spaces are fixable.
+	// After fix, MDS017 remains → formatDiagnostics is called in fixDiscovered.
+	writeFixture(t, dir, ".mdsmith.yml", "rules:\n  no-trailing-punctuation: true\n  no-trailing-spaces: true\n")
+	writeFixture(t, dir, "dirty.md", "# Title!\n\nHello   \n")
+
+	_, stderr, exitCode := runBinaryInDir(t, dir, "", "fix", "--no-color")
+	assert.Equal(t, 1, exitCode, "expected exit 1 (unfixable diagnostic), got %d", exitCode)
+	assert.Contains(t, stderr, "MDS017",
+		"expected MDS017 diagnostic in stderr, got: %s", stderr)
+}