Drop C409 tuple comprehension preview behaviour (#25707)
## Summary
Closes https://github.com/astral-sh/ruff/issues/12912 by only applying
the tuple generator behavior of the rule on Python 3.14+, where tuple
generator expression got optimized in (ref:
https://github.com/python/cpython/pull/131737). Rendering the concerns
on that issue no longer relevant on those Python versions.
This was especially problematic for `C409` as only some behaviour is in
preview, not the rule itself, so you couldn't use
`tool.ruff.lint.explicit-preview-rules = true`
## Test Plan
New snapshot should pass on CI
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Note: Code changed by Claude Sonnet 4.6. Reviewed by human, although I
don't know much about Rust (but this seems a simple enough change)
