[ty] introduce `DivergentType` (#20312)
## Summary
From #17371
In #17371, `DivergentType` was introduced to prevent type inference for
recursive functions from diverging and causing panics. This turned out
to be useful for other divergent type inferences
(https://github.com/astral-sh/ruff/pull/17371#discussion_r2329337965),
so I extracted the introduction part of `DivergentType` into this PR so
that we can use it without waiting for the merge of #17371.
Note that this PR only introduces `DivergentType` and does not actually
address divergent type inference yet. Please refer to
https://github.com/astral-sh/ruff/pull/17371/files#diff-20b910c6e20faa962bb1642e111db1cbad8e66ace089bdd966ac9d7f9fa99ff2R542-R622
etc. when implementing handling of divergence suppression using this
type.
## Test Plan
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Co-authored-by: Carl Meyer <carl@oddbird.net>
