fix: raise error when MlxJaccl requested without RDMA cycles (#1585)

## Summary
- When MlxJaccl (RDMA) placement is requested but no RDMA-connected
cycles exist, raise a clear ValueError instead of silently falling back
to non-RDMA cycles
- Split from #1519 for independent review

## Test plan
- [x] basedpyright — 0 errors
- [x] ruff check — passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <[email protected]>

src/exo/master/placement.py

@@ -112,7 +112,11 @@ def place_instance(
         cycle for cycle in smallest_cycles if topology.is_rdma_cycle(cycle)
     ]
 
-    if command.instance_meta == InstanceMeta.MlxJaccl and smallest_rdma_cycles != []:
+    if command.instance_meta == InstanceMeta.MlxJaccl:
+        if not smallest_rdma_cycles:
+            raise ValueError(
+                "Requested RDMA (MlxJaccl) but no RDMA-connected cycles available"
+            )
         smallest_cycles = smallest_rdma_cycles
 
     cycles_with_leaf_nodes: list[Cycle] = [