common : gracefully handle incomplete output (#20191)

* common : handle incomplete UTF-8 at end of input in PEG parser

* cont : if reached end prematurely, emit needs_more_input to propagate partial output

* cont: refactor peg parse context to add lenient flag

* cont : remove partial flag, keep lenient flag

tests/peg-parser/test-python-dict-parser.cpp

@@ -85,7 +85,7 @@ void test_python_dict_parser(testing &t) {
         auto parser = build_peg_parser([](common_peg_parser_builder & p) { return p.python_value(); });
 
         std::string    input = "{'name': 'test', 'value': ";
-        common_peg_parse_context ctx(input, true);
+        common_peg_parse_context ctx(input, COMMON_PEG_PARSE_FLAG_LENIENT);
 
         auto result = parser.parse(ctx);
 
@@ -97,7 +97,7 @@ void test_python_dict_parser(testing &t) {
         auto parser = build_peg_parser([](common_peg_parser_builder & p) { return p.python_value(); });
 
         std::string    input = "{'name': 'test";
-        common_peg_parse_context ctx(input, true);
+        common_peg_parse_context ctx(input, COMMON_PEG_PARSE_FLAG_LENIENT);
 
         auto result = parser.parse(ctx);
 
@@ -229,7 +229,7 @@ void test_python_dict_parser(testing &t) {
 
         t.test("incomplete string", [&](testing &t) {
             std::string input = "'hello";
-            common_peg_parse_context ctx(input, true);
+            common_peg_parse_context ctx(input, COMMON_PEG_PARSE_FLAG_LENIENT);
 
             auto result = parser.parse(ctx);
             t.assert_true("need_more_input", result.need_more_input());