fix(core/base_node): safe mmr_position cast in horizon sync (#5503)

Description
---
Perform safe numeric conversion of the `mmr_position` inside the horizon
state sync

Motivation and Context
---
There was an unsafe cast from `u64` to `u32` of the `mmr_position`
inside the horizon state sync. This PR does a safe cast, raising a typed
error instead of truncating the value.

The existing TODO comment suggested to do it inside the database
function, but in my opinion at this point this would overcomplicate the
solution.

How Has This Been Tested?
---
Unit tests

What process can a PR reviewer use to test or verify this change?
---

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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---------

Co-authored-by: stringhandler <stringhandler@gmail.com>
Co-authored-by: SW van Heerden <swvheerden@gmail.com>

base_layer/core/src/base_node/sync/horizon_state_sync/error.rs

@@ -65,6 +65,8 @@ pub enum HorizonSyncError {
         expected_hex: String,
         actual_hex: String,
     },
+    #[error("Invalid MMR position {mmr_position} at height {at_height}")]
+    InvalidMmrPosition { at_height: u64, mmr_position: u64 },
     #[error("Invalid range proof for output:{0} : {1}")]
     InvalidRangeProof(String, String),
     #[error("RPC error: {0}")]

base_layer/core/src/base_node/sync/horizon_state_sync/synchronizer.rs

@@ -319,11 +319,11 @@ impl<'a, B: BlockchainBackend + 'static> HorizonStateSynchronization<'a, B> {
 
             kernel_hashes.push(kernel.hash());
 
-            // TODO: We mix u32 and u64 for the mmr length. This comes down to the use of a 32-bit croaring Bitmap.
-            //       Suggest should use u64 externally (u64 is in the header) and error on the database calls if they
-            //       are > u32::MAX. Remove the clippy exception once fixed.
-            #[allow(clippy::cast_possible_truncation)]
-            txn.insert_kernel_via_horizon_sync(kernel, *current_header.hash(), mmr_position as u32);
+            let mmr_position_u32 = u32::try_from(mmr_position).map_err(|_| HorizonSyncError::InvalidMmrPosition {
+                at_height: current_header.height(),
+                mmr_position,
+            })?;
+            txn.insert_kernel_via_horizon_sync(kernel, *current_header.hash(), mmr_position_u32);
             if mmr_position == current_header.header().kernel_mmr_size - 1 {
                 let num_kernels = kernel_hashes.len();
                 debug!(