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/inline_assistant.rs

@@ -2732,7 +2732,7 @@ impl CodegenAlternative {
             messages,
             tools: Vec::new(),
             stop: Vec::new(),
-            temperature: 1.,
+            temperature: None,
         })
     }