[ty] Autocomplete arguments if in arguments node (#24167)
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## Summary
Addresses [this
comment](https://github.com/astral-sh/ty/issues/3087#issuecomment-4096258545).
Fixes https://github.com/astral-sh/ty/issues/3087.
I initially did the same ancestor walking check in both places but then
I tried to come up with another way that does not iterate multiple
times.
Since in `add_argument_completions` we are already iterating over node
ancestors of the node the cursor is in, we can determine if cursor is in
an arguments node.
So I did that instead of duplicating the
`cursor.covering_node.ancestors() ...` code.
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## Test Plan
Added the test case that would panic in debug build without this fix.
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