fix(java_opts): use string-split to restore JAVA_OPTS placeholder

bash 4.x and 5.x treat backslash in ${//} replacement strings differently:
bash 5.x collapses '\\' to '\', bash 4.x leaves it as '\\'. A JAVA_OPTS
value containing a single backslash (e.g. -Dpattern=foo\|bar) was therefore
doubled on CI workers running bash 4.x, causing the unit test
'preserves backslashes in JAVA_OPTS values' to fail remotely while
passing locally on bash 5.x.

Replace the pre-escaped ${//} substitution with string-split using
${%%} and ${#} — pure string operations that never interpret backslash
or '&', identical across bash 3/4/5.

Author: stokpop

src/java/frameworks/java_opts_writer.go

@@ -141,13 +141,13 @@ case "$USER_JAVA_OPTS" in
 esac
 
 # Escape replacement-special chars once; these values are loop-invariant.
+# USER_JAVA_OPTS is injected via string-split below (not ${//}) — bash 4.x and 5.x
+# treat '\\' in replacement strings differently, corrupting backslashes.
 _escaped_deps_dir="${DEPS_DIR//\\/\\\\}"
 _escaped_deps_dir="${_escaped_deps_dir//&/\\&}"
 _escaped_home="${HOME//\\/\\\\}"
 _escaped_home="${_escaped_home//&/\\&}"
 _user_java_opts_placeholder='__JAVA_OPTS_BUILDPACK_PLACEHOLDER__'
-_escaped_user_java_opts="${USER_JAVA_OPTS//\\/\\\\}"
-_escaped_user_java_opts="${_escaped_user_java_opts//&/\\&}"
 
 if [ -d "$DEPS_DIR/%s/java_opts" ]; then
     for opts_file in "$DEPS_DIR/%s/java_opts"/*.opts; do
@@ -194,8 +194,16 @@ if [ -d "$DEPS_DIR/%s/java_opts" ]; then
                     ;;
             esac
 
-            # Now safely substitute JAVA_OPTS after expansion (preserves quotes, backslashes, and ampersands)
-            opts_content="${opts_content//$_user_java_opts_placeholder/$_escaped_user_java_opts}"
+            # Restore USER_JAVA_OPTS via string-split, not ${//}: bash 4.x and 5.x
+            # treat '\\' in replacement strings differently, corrupting backslashes.
+            # %% / # are pure string ops — no special-char interpretation, any bash.
+            # Guard required: without placeholder present, %% and # return the full
+            # string, so omitting the guard duplicates opts_content.
+            if [[ "$opts_content" == *"$_user_java_opts_placeholder"* ]]; then
+                _before="${opts_content%%"$_user_java_opts_placeholder"*}"
+                _after="${opts_content#*"$_user_java_opts_placeholder"}"
+                opts_content="${_before}${USER_JAVA_OPTS}${_after}"
+            fi
             
             if [ -n "$opts_content" ]; then
                 JAVA_OPTS="$JAVA_OPTS $opts_content"