fix: api ergonomics follow-up (#720)

* fix: builder with_* methods take T instead of Option<T>

* fix: emit conditional builder calls for optional fields in macros

* fix: convert with_task, with_stop_reason, with_logger, with_content to proper builders

* fix: update test callers for new builder signatures

* fix: simplify make_task helper and remove unused import

* fix: update sampling_stdio example for new with_stop_reason signature

* fix: make annotations and execution Option<Expr> consistent with other fields

* fix: remove unused none_expr import

examples/clients/src/sampling_stdio.rs

@@ -44,7 +44,7 @@ impl ClientHandler for SamplingDemoClient {
             SamplingMessage::assistant_text(response_text),
             "mock_llm".to_string(),
         )
-        .with_stop_reason(Some(CreateMessageResult::STOP_REASON_END_TURN.to_string())))
+        .with_stop_reason(CreateMessageResult::STOP_REASON_END_TURN))
     }
 }