fix: guard repeat array length overflow

[Sean-Kenneth-Doherty]

Which issue does this PR close?

• Closes panic: repeat string array path overflows capacity calculation #22217.

Rationale for this change

The array execution path for repeat(string, count) calculated string.len() * count before checking the configured string-size limit. For very large counts, that multiplication can overflow and panic instead of returning the same string-size overflow error used by the scalar path.

What changes are included in this PR?

• Adds checked count conversion and repeated-length calculation helpers.
• Uses checked multiplication and checked total-capacity accumulation in the array path.
• Adds Rust and sqllogictest coverage for the one-row columnar reproducer from the issue.

Are these changes tested?

• cargo fmt --all
• TMPDIR=/home/sean/Projects/datafusion-repeat-overflow/target/tmp cargo test -p datafusion-functions string::repeat::tests::test_repeat_string_array_overflow -- --nocapture
• TMPDIR=/home/sean/Projects/datafusion-repeat-overflow/target/tmp cargo test --profile=ci --test sqllogictests -- string/string_literal.slt
• TMPDIR=/home/sean/Projects/datafusion-repeat-overflow/target/tmp cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Are there any user-facing changes?

Invalid oversized repeat results in the columnar path now return a normal DataFusion string-size overflow error instead of panicking.

[Sean-Kenneth-Doherty]

Fresh local validation on be1f0357b:

• cargo test -p datafusion-functions string::repeat::tests::test_repeat_string_array_overflow -> passed
• cargo test -p datafusion-sqllogictest --test sqllogictests -- string/string_literal.slt -> passed
• cargo test -p datafusion-functions -> 283 unit tests passed plus 3 doctests passed / 4 doctests ignored
• cargo fmt --all --check -> passed
• cargo clippy -p datafusion-functions --all-targets -- -D warnings -> passed
• git diff --check origin/main...HEAD -> clean

GitHub currently only shows the labeler Process check, so this adds local coverage for the changed repeat overflow path and the touched SQLLogicTest file.