unconditionally disable mypy db sharding - #23578
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mypy 2.0 <https://mypy-lang.blogspot.com/2026/05/mypy-20-relased.html> began sharding the sqlite files (so instead of one .db file you get 16 or whatever). This breaks the mypy<-->named_cache integration which is designed around managing a single file atomically. To prevent users seeing a 1/num-shards performance degradation, unconditionally use 1 shard and warn on attempts to overrides. NOTE: The warning is best effort and doesn't try to handle the setting being in mypy config files. closes pantsbuild#23352
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mypy 2.0 https://mypy-lang.blogspot.com/2026/05/mypy-20-relased.html began sharding the sqlite files (so instead of one .db file you get 16 or whatever). This breaks the mypy<-->named_cache integration which is designed around managing a single file atomically. To prevent users seeing a 1/num-shards performance degradation, unconditionally use 1 shard and warn on attempts to overrides.
NOTE: The warning is best effort and doesn't try to handle the setting being in mypy config files.
closes #23352