chat: harden peg-native tool call parsing (#24329)

* chat: harden peg-native tool call parsing

accept an optional leading type: function field in
build_json_tools_flat_keys so openai style tool calls parse on
templates whose serialization opens on the name field.

return a clean error and log the unparsed fragment on a final peg
parse failure instead of throwing the raw parser position and input.

keep the raw arguments string in func_args_not_string when it is not
valid json instead of aborting the prompt render.

* chat: surface peg-native parse failures

a final peg parse failure threw the raw parser position and input. log
the unparsed fragment and raise a clearer error instead, so a model
output that does not match the expected format no longer fails silently
with an empty assistant turn.

minimal change, no behavior change on successful parses.

* chat: handle openai style tool calls in peg-native

* nits

* common: scope OpenAI wrapper grammar trigger via autoparser flag

* chat: gate type:function parsing leniency on the analysis flag

Thread accept_openai_wrapper from the generator to build_json_tools_flat_keys
so the leading "type": "function" field is accepted only when openai_wrapper_trigger is set.

common/chat-auto-parser.h

@@ -181,6 +181,7 @@ struct tool_format_analysis {
 
     bool fun_name_is_key = false;       // In JSON format function name is JSON key, i.e. { "<funname>": { ... arguments ... } }
     bool tools_array_wrapped = false;   // Tool calls wrapped in JSON array [...]
+    bool openai_wrapper_trigger = false;  // model emits the OpenAI function wrapper, trigger on it
 
     std::string              function_field = "function";
     std::string              name_field     = "name";