Allow Anthropic custom models to override temperature (#18160)

Release Notes:

- Allow Anthropic custom models to override "temperature"

This also centralized the defaulting of "temperature" to be inside of
each model's `into_x` call instead of being sprinkled around the code.

crates/assistant/src/slash_command/auto_command.rs

@@ -216,7 +216,7 @@ async fn commands_for_summaries(
         }],
         tools: Vec::new(),
         stop: Vec::new(),
-        temperature: 1.0,
+        temperature: None,
     };
 
     while let Some(current_summaries) = stack.pop() {