common : implement common_schema internal representation for JSON schemas (#28736)

* common : implement common_schema types

* common : implement a json schema optimizer

* common : reduce optimizations

* common : refactor json-schema-to-grammar to use common_schema

* common : use common_trie

* common/schema : implement type/kind resolution

* cont : cleanup

* cont : remove common_chat_tool_parameters

* cont : simplify schema resolution

* cont : pass common_schema through the json-schema-to-grammar builder

* cont : cleanup

* cont : move enums under common_schema and add type enum

* cont : reduce test cases

* cont : clean up

* cont : clean up

* refactor : rename common_schema_parse to common_schema_from_json

* tests : fix gcc dangling-reference warning in test-json-schema

* tests : take the schema label as const char * to satisfy gcc dangling-reference

* refactor : rename common_schema_builder parse_* methods to build_*

* cont : fix may_be_string

* cont : properly handle empty tool parameters

* cont : add tests for empty $ref

* cont : remove dead code

* cont : update docs

* cont : make "{}" mean any object for json_object as well

* cont : restore (min|max)Length to imply string type

* cont : rename common_schema to common_chat_schema

tests/test-chat.cpp

@@ -472,6 +472,12 @@ static common_chat_tool empty_args_tool_no_properties{
     })",
 };
 
+static common_chat_tool empty_args_tool_no_schema{
+    /* .name = */ "empty_args_no_schema",
+    /* .description = */ "A tool that takes no arguments and has no parameters schema",
+    /* .parameters = */ "{}",
+};
+
 static common_chat_tool python_tool{
     /* .name = */ "python",
     /* .description = */ "an ipython interpreter",
@@ -5071,6 +5077,13 @@ static void test_template_output_peg_parsers(bool detailed_debug) {
             .expect(simple_assist_msg("", "", "empty_args", "{}"))
             .run();
 
+        // Tool call with no parameters schema, {} means no arguments
+        tst.test("<tool_call>\n{\"name\": \"empty_args_no_schema\", \"arguments\": {}}</tool_call>")
+            .enable_thinking(false)
+            .tools({ empty_args_tool_no_schema })
+            .expect(simple_assist_msg("", "", "empty_args_no_schema", "{}"))
+            .run();
+
         // fake tool call marker in reasoning
         tst.test(
                "Let me think about <tool_call>\n{\"name\": \"special_function\", \"arguments\": {\"arg1\": 2}}</tool_call> hmm\n</think>\n\n"