feat(transport): add constructors for non_exhaustive error types (#806)

AuthRequiredError, InsufficientScopeError, and DynamicTransportError
were marked #[non_exhaustive] in #715/#768 but don't have constructors
usable by external crates. Add new() for the error types and
from_parts() for DynamicTransportError (the existing new() requires a
Transport type parameter, making it unusable for test fixtures).

Fixes #805

crates/rmcp/src/transport.rs

@@ -252,6 +252,24 @@ impl DynamicTransportError {
             error: Box::new(e),
         }
     }
+
+    /// Create a `DynamicTransportError` from raw parts.
+    ///
+    /// Unlike [`new`](Self::new), this does not require a concrete [`Transport`] type,
+    /// making it usable in test fixtures and other contexts where a real transport
+    /// implementation is not available.
+    pub fn from_parts(
+        transport_name: impl Into<Cow<'static, str>>,
+        transport_type_id: std::any::TypeId,
+        error: Box<dyn std::error::Error + Send + Sync>,
+    ) -> Self {
+        Self {
+            transport_name: transport_name.into(),
+            transport_type_id,
+            error,
+        }
+    }
+
     pub fn downcast<T: Transport<R> + 'static, R: ServiceRole>(self) -> Result<T::Error, Self> {
         if !self.is::<T, R>() {
             Err(self)

crates/rmcp/src/transport/common/reqwest/streamable_http_client.rs

@@ -284,21 +284,28 @@ impl StreamableHttpClientTransport<reqwest::Client> {
 #[cfg(test)]
 mod tests {
     use super::parse_json_rpc_error;
-    use crate::{model::JsonRpcMessage, transport::streamable_http_client::InsufficientScopeError};
+    use crate::{
+        model::JsonRpcMessage,
+        transport::streamable_http_client::{AuthRequiredError, InsufficientScopeError},
+    };
+
+    #[test]
+    fn auth_required_error_new() {
+        let err = AuthRequiredError::new("Bearer realm=\"test\"".to_string());
+        assert_eq!(err.www_authenticate_header, "Bearer realm=\"test\"");
+    }
 
     #[test]
     fn insufficient_scope_error_can_upgrade() {
-        let with_scope = InsufficientScopeError {
-            www_authenticate_header: "Bearer scope=\"admin\"".to_string(),
-            required_scope: Some("admin".to_string()),
-        };
+        let with_scope = InsufficientScopeError::new(
+            "Bearer scope=\"admin\"".to_string(),
+            Some("admin".to_string()),
+        );
         assert!(with_scope.can_upgrade());
         assert_eq!(with_scope.get_required_scope(), Some("admin"));
 
-        let without_scope = InsufficientScopeError {
-            www_authenticate_header: "Bearer error=\"insufficient_scope\"".to_string(),
-            required_scope: None,
-        };
+        let without_scope =
+            InsufficientScopeError::new("Bearer error=\"insufficient_scope\"".to_string(), None);
         assert!(!without_scope.can_upgrade());
         assert_eq!(without_scope.get_required_scope(), None);
     }

crates/rmcp/src/transport/streamable_http_client.rs

@@ -29,6 +29,15 @@ pub struct AuthRequiredError {
     pub www_authenticate_header: String,
 }
 
+impl AuthRequiredError {
+    /// Create a new `AuthRequiredError` instance.
+    pub fn new(www_authenticate_header: String) -> Self {
+        Self {
+            www_authenticate_header,
+        }
+    }
+}
+
 #[derive(Debug)]
 #[non_exhaustive]
 pub struct InsufficientScopeError {
@@ -37,6 +46,14 @@ pub struct InsufficientScopeError {
 }
 
 impl InsufficientScopeError {
+    /// Create a new `InsufficientScopeError` instance.
+    pub fn new(www_authenticate_header: String, required_scope: Option<String>) -> Self {
+        Self {
+            www_authenticate_header,
+            required_scope,
+        }
+    }
+
     /// check if scope upgrade is possible (i.e., we know what scope is required)
     pub fn can_upgrade(&self) -> bool {
         self.required_scope.is_some()