Fix tool-call parsing to ignore non-tool JSON (fixes #511)

[markknoffler]

Fixes: #511

Summary

• Prevent OneShotToolManager from mis-parsing arbitrary JSON in model output as a tool call.
• Avoids KeyError: 'tool_name' when the model outputs normal JSON (e.g., structured responses) that is not a tool call.

What changed

• Updated gemma/gm/tools/_manager.py to:
  • Stop using greedy brace-regex extraction for JSON.
  • Only treat parsed JSON as a tool call when it is a JSON object containing a valid string tool_name.
  • Gracefully ignore non-tool JSON by returning None instead of raising.

Before / After

• Before: any {...} JSON in model output could be parsed as a tool call → KeyError: 'tool_name' if missing.
• After: non-tool JSON is ignored; only JSON objects containing tool_name are executed as tool calls.