chore: add #[non_exhaustive] and mutation methods to improve compatibility (#715)

* chore: add #[non_exhaustive] to reduce backwards-incompatible changes going forward

* fix: remove ProtocolVersion import

* fix: add a few more with_ mutator methods

---------

Co-authored-by: Alex Hancock <alexhancock@block.xyz>

crates/rmcp/src/model/task.rs

@@ -28,6 +28,7 @@ pub enum TaskStatus {
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, Default)]
 #[serde(rename_all = "camelCase")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[non_exhaustive]
 pub struct Task {
     /// Unique task identifier generated by the receiver.
     pub task_id: String,
@@ -48,14 +49,60 @@ pub struct Task {
     pub poll_interval: Option<u64>,
 }
 
+impl Task {
+    /// Create a new Task with required fields.
+    pub fn new(
+        task_id: String,
+        status: TaskStatus,
+        created_at: String,
+        last_updated_at: String,
+    ) -> Self {
+        Self {
+            task_id,
+            status,
+            status_message: None,
+            created_at,
+            last_updated_at,
+            ttl: None,
+            poll_interval: None,
+        }
+    }
+
+    /// Set the status message.
+    pub fn with_status_message(mut self, status_message: impl Into<String>) -> Self {
+        self.status_message = Some(status_message.into());
+        self
+    }
+
+    /// Set the TTL in milliseconds. `None` means unlimited retention.
+    pub fn with_ttl(mut self, ttl: u64) -> Self {
+        self.ttl = Some(ttl);
+        self
+    }
+
+    /// Set the poll interval in milliseconds.
+    pub fn with_poll_interval(mut self, poll_interval: u64) -> Self {
+        self.poll_interval = Some(poll_interval);
+        self
+    }
+}
+
 /// Wrapper returned by task-augmented requests (CreateTaskResult in SEP-1686).
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[non_exhaustive]
 pub struct CreateTaskResult {
     pub task: Task,
 }
 
+impl CreateTaskResult {
+    /// Create a new CreateTaskResult.
+    pub fn new(task: Task) -> Self {
+        Self { task }
+    }
+}
+
 /// Response to a `tasks/get` request.
 ///
 /// Per spec, `GetTaskResult = allOf[Result, Task]` — the Task fields are
@@ -78,8 +125,16 @@ pub struct GetTaskResult {
 /// serialized as a JSON value.
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+#[non_exhaustive]
 pub struct GetTaskPayloadResult(pub Value);
 
+impl GetTaskPayloadResult {
+    /// Create a new GetTaskPayloadResult with the given value.
+    pub fn new(value: Value) -> Self {
+        Self(value)
+    }
+}
+
 /// Response to a `tasks/cancel` request.
 ///
 /// Per spec, `CancelTaskResult = allOf[Result, Task]` — same shape as `GetTaskResult`.
@@ -104,3 +159,14 @@ pub struct TaskList {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub total: Option<u64>,
 }
+
+impl TaskList {
+    /// Create a new TaskList.
+    pub fn new(tasks: Vec<Task>) -> Self {
+        Self {
+            tasks,
+            next_cursor: None,
+            total: None,
+        }
+    }
+}