test: avoid broken pipe in template-loader assertions

Use here-strings instead of echo | grep pipelines so that grep -q
finishing early cannot raise SIGPIPE on echo under set -o pipefail.
Mirrors the same fix already applied to test-templates-comprehensive.sh.

Author: SihaoLiu

tests/test-template-loader.sh

@@ -155,9 +155,9 @@ RESULT=$(load_and_render "$TEMPLATE_DIR" "block/wrong-round-number.md" \
     "CURRENT_ROUND=5" \
     "CORRECT_PATH=/tmp/round-5-summary.md")
 
-if echo "$RESULT" | grep -q "Wrong Round Number" && \
-   echo "$RESULT" | grep -q "round-3-summary.md" && \
-   echo "$RESULT" | grep -q "current round is \*\*5\*\*"; then
+if grep -q "Wrong Round Number" <<<"$RESULT" && \
+   grep -q "round-3-summary.md" <<<"$RESULT" && \
+   grep -q "current round is \*\*5\*\*" <<<"$RESULT"; then
     pass "load_and_render works correctly with real template"
 else
     fail "load_and_render integration test" "Content with replaced variables" "$RESULT"
@@ -201,7 +201,7 @@ echo "Test 11: load_and_render_safe - missing template uses fallback"
 FALLBACK="Fallback message: {{VAR}}"
 RESULT=$(load_and_render_safe "$TEMPLATE_DIR" "non-existing.md" "$FALLBACK" "VAR=test_value")
 
-if echo "$RESULT" | grep -q "Fallback message: test_value"; then
+if grep -q "Fallback message: test_value" <<<"$RESULT"; then
     pass "load_and_render_safe uses fallback for missing template"
 else
     fail "load_and_render_safe fallback" "Fallback message: test_value" "$RESULT"
@@ -215,7 +215,7 @@ echo "Test 12: load_and_render_safe - existing template works normally"
 FALLBACK="This should not appear"
 RESULT=$(load_and_render_safe "$TEMPLATE_DIR" "block/git-push.md" "$FALLBACK")
 
-if echo "$RESULT" | grep -q "Git Push Blocked" && ! echo "$RESULT" | grep -q "should not appear"; then
+if grep -q "Git Push Blocked" <<<"$RESULT" && ! grep -q "should not appear" <<<"$RESULT"; then
     pass "load_and_render_safe uses template when available"
 else
     fail "load_and_render_safe with existing template" "Git Push Blocked (not fallback)" "$RESULT"