Add fast mode toggle in agent panel (#49714)

This is a staff only toggle for now, since the consequences of
activating it are not obvious and quite dire (tokens costs 6 times
more).

Also, persist thinking, thinking effort and fast mode in DbThread so the
thinking mode toggle and thinking effort are persisted.

Release Notes:

- Agent: The thinking mode toggle and thinking effort are now persisted
when selecting a thread from history.

crates/agent_ui/src/buffer_codegen.rs

@@ -547,6 +547,7 @@ impl CodegenAlternative {
                 messages,
                 thinking_allowed: false,
                 thinking_effort: None,
+                speed: None,
             }
         }))
     }
@@ -626,6 +627,7 @@ impl CodegenAlternative {
                 messages: vec![request_message],
                 thinking_allowed: false,
                 thinking_effort: None,
+                speed: None,
             }
         }))
     }