In #129825, we modified the dense_vector field type to delay setting index options until the field's dimensions are known. However, this introduced a discrepancy for indices created before that change, which would previously default to int8_hnsw even when dimensions were not set.
This discrepancy leads to an assertion failure in mixed-version clusters, where the serialized mappings differ between nodes:
```
[2025-07-02T20:37:29,852][ERROR][o.e.b.ElasticsearchUncaughtExceptionHandler] [v9.0.4-2] fatal error in thread [elasticsearch[v9.0.4-2][clusterApplierService#updateTask][T#1]], exiting java.lang.AssertionError: provided source [{"_doc":{"properties":{"vector":{"type":"dense_vector","index":true,"similarity":"cosine"}}}}] differs from mapping [{"_doc":{"properties":{"vector":{"type":"dense_vector","index":true,"similarity":"cosine","index_options":{"type":"int8_hnsw","m":16,"ef_construction":100}}}}}]
```
This commit resolves the issue by ensuring that indices created before the change continue to default to int8_hnsw index options, even if dimensions remain unset.
f91124a59e