Fix UBSan abort caused by integer overflow when calculating time difference
between read and write operations. The issue occurs when:
1. The queried partition on replicas is not purgeable (has no recorded
modified time)
2. Digests don't match across replicas
3. The system attempts to calculate timespan using missing/negative
last_modified timestamps
This change skips cross-DC repair optimization when write timestamp is
negative or missing, as this optimization is only relevant for reads
occurring within write_timeout of a write.
Error details:
```
service/storage_proxy.cc:5532:80: runtime error: signed integer overflow: -9223372036854775808 - 1741940132787203 cannot be represented in type 'int64_t' (aka 'long')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior service/storage_proxy.cc:5532:80
Aborting on shard 1, in scheduling group sl:default
```
Related to previous fix 39325cf which handled negative read_timestamp cases.
Fixes#23314
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#23359
ebf9125728