[ty] Avoid duplicate diagnostics for overloaded TypeIs (#26716)
## Summary
Check only the current function definition when validating `TypeIs` and
`TypeGuard` definitions. `check_type_guard_definition` runs for every
`FunctionDef`, which includes each individual overload, so it shouldn't
iterate previous overloads each time also.
Fixes astral-sh/ty#3968
## Validation
Added mdtests.
