ddl: use max(approximate_size, approximate_kv_size) for add-index table size estimation

## Feature Request\n\n**Is your feature request related to a problem? Please describe:**\n\nTiDB DDL add-index table size estimation currently depends on PD region size fields. When and diverge, using only one field can underestimate table size and affect resource/concurrency decisions.\n\n**Describe the feature you'd like:**\n\nUse per region in DDL table size estimation () to avoid underestimation.\n\n**Describe alternatives you've considered:**\n\n1. Prefer with fallback to when kv size is 0.\n2. Prefer only.\n\nBoth can underestimate when both fields are present but one is lower than the other.\n\n**Teachability, Documentation, Adoption, Migration Strategy:**\n\nNo SQL syntax or user-facing configuration changes. Behavior improvement is internal to DDL estimation and covered by targeted unit tests.