Thought signatures (#42915)

Implement Gemini API's [thought
signatures](https://ai.google.dev/gemini-api/docs/thinking#signatures)

Release Notes:

- Added thought signatures for Gemini tool calls

crates/agent/src/edit_agent/evals.rs

@@ -1108,6 +1108,7 @@ fn tool_use(
         raw_input: serde_json::to_string_pretty(&input).unwrap(),
         input: serde_json::to_value(input).unwrap(),
         is_input_complete: true,
+        thought_signature: None,
     })
 }