Send only blocks at missing heights (#5206)

## Motivation

When syncing chain information for blobs between validators, we were
sending more data than necessary. The previous implementation would send
**all blocks up to the height** where a blob was published, even when
only the specific block containing that blob was needed.

For sparse chains (chains with gaps in block heights), this meant
sending many unnecessary blocks, wasting network bandwidth and validator
processing time.

Addresses
#5205 (comment)

  ## Proposal

This PR changes the blob synchronization strategy from "send all blocks
up to height N" to "send only blocks at specific heights":

1. **Modified `send_chain_info_for_blobs`**: Instead of tracking the
maximum height per chain, now collects a `BTreeSet` of all specific
heights where blobs exist
2. **New method `send_chain_info_at_heights`**: Sends chain information
only for the exact block heights specified, not all blocks leading up to
those heights
3. **Updated documentation**: Clarifies that this optimization
specifically benefits sparse chains

  ### Example Impact

  Previously, if blobs existed at heights [5, 10, 15] on a chain:
- Sent blocks 0-5, 0-10, 0-15 (up to 30 blocks sent, with significant
duplication)

  Now:
  - Sends only blocks at heights 5, 10, 15 (exactly 3 blocks)

  ## Test Plan

  - CI

  ## Release Plan

  - Nothing to do / These changes follow the usual release cycle.
  - Backport to `main`.

  ## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: deuszx

linera-core/src/updater.rs

@@ -662,6 +662,7 @@ where
     /// # Returns
     /// - `Ok(())` if synchronization completed successfully or the validator is already up to date
     /// - `Err` if there was a communication or storage error
+    #[instrument(level = "trace", skip_all)]
     pub async fn send_chain_information(
         &mut self,
         chain_id: ChainId,
@@ -896,8 +897,9 @@ where
 
     /// Sends chain information for all chains referenced by the given blobs.
     ///
-    /// Reads blob states from storage, determines the chain heights needed,
-    /// and sends chain information to bring the validator up to date.
+    /// Reads blob states from storage, determines the specific chain heights needed,
+    /// and sends chain information for those heights. With sparse chains, this only
+    /// sends the specific blocks containing the blobs, not all blocks up to those heights.
     async fn send_chain_info_for_blobs(
         &mut self,
         blob_ids: &[BlobId],
@@ -909,20 +911,72 @@ where
             .read_blob_states_from_storage(blob_ids)
             .await?;
 
-        let mut chain_heights = BTreeMap::new();
+        let mut chain_heights: BTreeMap<ChainId, BTreeSet<BlockHeight>> = BTreeMap::new();
         for blob_state in blob_states {
             let block_chain_id = blob_state.chain_id;
-            let block_height = blob_state.block_height.try_add_one()?;
+            let block_height = blob_state.block_height;
             chain_heights
                 .entry(block_chain_id)
-                .and_modify(|h| *h = block_height.max(*h))
-                .or_insert(block_height);
+                .or_default()
+                .insert(block_height);
         }
 
-        self.send_chain_info_up_to_heights(chain_heights, delivery)
+        self.send_chain_info_at_heights(chain_heights, delivery)
             .await
     }
 
+    /// Sends chain information for specific heights on multiple chains.
+    ///
+    /// Unlike `send_chain_info_up_to_heights`, this method only sends the blocks at the
+    /// specified heights, not all blocks up to those heights. This is more efficient for
+    /// sparse chains where only specific blocks are needed.
+    async fn send_chain_info_at_heights(
+        &mut self,
+        chain_heights: impl IntoIterator<Item = (ChainId, BTreeSet<BlockHeight>)>,
+        delivery: CrossChainMessageDelivery,
+    ) -> Result<(), ChainClientError> {
+        FuturesUnordered::from_iter(chain_heights.into_iter().map(|(chain_id, heights)| {
+            let mut updater = self.clone();
+            async move {
+                // Get all block hashes for this chain at the specified heights in one call
+                let heights_vec: Vec<_> = heights.into_iter().collect();
+                let hashes = updater
+                    .client
+                    .local_node
+                    .get_block_hashes(chain_id, heights_vec.clone())
+                    .await?;
+
+                if hashes.len() != heights_vec.len() {
+                    return Err(ChainClientError::InternalError(
+                        "send_chain_info_at_heights called with invalid heights",
+                    ));
+                }
+
+                // Read all certificates in one call
+                let certificates = updater
+                    .client
+                    .local_node
+                    .storage_client()
+                    .read_certificates(hashes.clone())
+                    .await?;
+
+                // Send each certificate
+                for (hash, certificate) in hashes.into_iter().zip(certificates) {
+                    let certificate =
+                        certificate.ok_or_else(|| ChainClientError::MissingConfirmedBlock(hash))?;
+                    updater
+                        .send_confirmed_certificate(certificate, delivery)
+                        .await?;
+                }
+
+                Ok::<_, ChainClientError>(())
+            }
+        }))
+        .try_collect::<Vec<_>>()
+        .await?;
+        Ok(())
+    }
+
     async fn send_chain_info_up_to_heights(
         &mut self,
         chain_heights: impl IntoIterator<Item = (ChainId, BlockHeight)>,