Delete RuntimeVariableList::from_vec (#7930)

This method is a footgun because it truncates the list. It is the source of a recent bug:

- https://github.com/sigp/lighthouse/pull/7927


  - Delete uses of `RuntimeVariableList::from_vec` and replace them with `::new` which does validation and can fail.
- Propagate errors where possible, unwrap in tests and use `expect` for obviously-safe uses (in `chain_spec.rs`).

beacon_node/network/src/sync/network_context/requests/blobs_by_root.rs

@@ -11,7 +11,7 @@ pub struct BlobsByRootSingleBlockRequest {
 }
 
 impl BlobsByRootSingleBlockRequest {
-    pub fn into_request(self, spec: &ForkContext) -> BlobsByRootRequest {
+    pub fn into_request(self, spec: &ForkContext) -> Result<BlobsByRootRequest, String> {
         BlobsByRootRequest::new(
             self.indices
                 .into_iter()

beacon_node/network/src/sync/network_context/requests/blocks_by_root.rs

@@ -9,7 +9,8 @@ use super::{ActiveRequestItems, LookupVerifyError};
 pub struct BlocksByRootSingleRequest(pub Hash256);
 
 impl BlocksByRootSingleRequest {
-    pub fn into_request(self, fork_context: &ForkContext) -> BlocksByRootRequest {
+    pub fn into_request(self, fork_context: &ForkContext) -> Result<BlocksByRootRequest, String> {
+        // This should always succeed (single block root), but we return a `Result` for safety.
         BlocksByRootRequest::new(vec![self.0], fork_context)
     }
 }

beacon_node/network/src/sync/network_context/requests/data_columns_by_root.rs

@@ -21,13 +21,13 @@ impl DataColumnsByRootSingleBlockRequest {
     ) -> Result<DataColumnsByRootRequest<E>, &'static str> {
         let columns = VariableList::new(self.indices)
             .map_err(|_| "Number of indices exceeds total number of columns")?;
-        Ok(DataColumnsByRootRequest::new(
+        DataColumnsByRootRequest::new(
             vec![DataColumnsByRootIdentifier {
                 block_root: self.block_root,
                 columns,
             }],
             spec.max_request_blocks(fork_name),
-        ))
+        )
     }
 }