[Consensus] fix GC missing blocks reported via the try_fetch_blocks m…

…ethod (#20992)

## Description 

Blocks reported as missing via the `block_manager.try_fetch_blocks`
method because they are not added in the `missing_ancestors` map they
are not picked up for cleaning via the Garbage Collection mechanism.
Those blocks are then picked up by the periodic synchronizer to fetch -
which is unnecessary - and even worse, since the majority of the network
might have already rejected them there aren't many many authorities
around which actually have the blocks available to serve. That leads to
the periodic synchronizer constantly working and not able to fetch any
of those blocks.

This PR is fixing this behaviour by adding the blocks in the
`missing_ancestors` map so they are properly picked up by GC for clean
up.

## Test plan 

Wrote a test which confirmed the existing behaviour (blocks are stuck in
the block manager and not cleaned up). Then when applied the fix test
turned into Green.

CI/PT

---

## Release notes

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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] gRPC:
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:

consensus/core/src/block_manager.rs

@@ -170,6 +170,10 @@ impl BlockManager {
             // Fetches the block if it is not in dag state or suspended.
             missing_blocks.insert(*block_ref);
             if self.missing_blocks.insert(*block_ref) {
+                // We want to report this as a missing ancestor even if there is no block that is actually references it right now. That will allow us
+                // to seamlessly GC the block later if needed.
+                self.missing_ancestors.entry(*block_ref).or_default();
+
                 let block_ref_hostname =
                     &self.context.committee.authority(block_ref.author).hostname;
                 self.context
@@ -994,6 +998,14 @@ mod tests {
             }
             assert!(!block_manager.is_empty());
 
+            // AND also call the try_to_find method with some non existing block refs. Those should be cleaned up as well once GC kicks in.
+            let non_existing_refs = (1..=3)
+                .map(|round| {
+                    BlockRef::new(round, AuthorityIndex::new_for_test(0), BlockDigest::MIN)
+                })
+                .collect::<Vec<_>>();
+            assert_eq!(block_manager.try_find_blocks(non_existing_refs).len(), 3);
+
             // AND
             // Trigger a commit which will advance GC round
             let last_commit = TrustedCommit::new_for_test(