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/context.rs

@@ -2180,7 +2180,7 @@ impl Context {
             messages: Vec::new(),
             tools: Vec::new(),
             stop: Vec::new(),
-            temperature: 1.0,
+            temperature: None,
         };
         for message in self.messages(cx) {
             if message.status != MessageStatus::Done {