[ty] Add `Final` mdtests for loops and redeclaration (#23331)
## Summary
Fixes https://github.com/astral-sh/ty/issues/2689
Update `Final` mdtests to reflect current policy for loop usage and
same-scope redeclaration, and document a known TODO loop limitation.
## Test Plan
- Module-level `Final` reassignment in a loop errors
- `Final` declarations in loop bodies are accepted
- Same-scope `Final` redeclaration is accepted
- Known `Final`-in-loop limitation is documented via TODO mdtest
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Co-authored-by: Carl Meyer <carl@oddbird.net>
