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/service.rs

@@ -566,6 +566,7 @@ impl RunningServiceCancellationToken {
 }
 
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum QuitReason {
     Cancelled,
     Closed,
@@ -584,6 +585,19 @@ pub struct RequestContext<R: ServiceRole> {
     pub peer: Peer<R>,
 }
 
+impl<R: ServiceRole> RequestContext<R> {
+    /// Create a new RequestContext.
+    pub fn new(id: RequestId, peer: Peer<R>) -> Self {
+        Self {
+            ct: CancellationToken::new(),
+            id,
+            meta: Meta::default(),
+            extensions: Extensions::default(),
+            peer,
+        }
+    }
+}
+
 /// Request execution context
 #[derive(Debug, Clone)]
 pub struct NotificationContext<R: ServiceRole> {