fix: prevent uncaught JSONDecodeError in tryparse() fallback

[Dabao21]

Problem

In llmcore.py, the tryparse() function attempts multiple strategies to recover malformed JSON from LLM output. However, the final json.loads() call after trimming to the last } is not wrapped in a try/except block:

if '}' in json_str: json_str = json_str[:json_str.rfind('}') + 1]
return json.loads(json_str)  # ← can still throw JSONDecodeError

When all recovery attempts fail, this raises an uncaught JSONDecodeError that crashes the tool call parser.

Fix

Wrap the final json.loads() in a try/except, returning {"_raw": json_str} on failure. This is consistent with the behavior of _try_parse_tool_args() which also returns a fallback dict when parsing fails.

Impact

• Prevents agent loop crashes when LLM returns completely unparseable tool call output
• The {"_raw": ...} fallback allows downstream code to still access the raw text for debugging