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)