What

Changes size() condition expression function to use UTF-16 code unit count for strings instead of UTF-8 byte length.

Why

Closes #60

DynamoDB uses UTF-16 code units for size() on strings. ExtendDB was using Rust's str::len() (UTF-8 bytes), causing condition expressions to evaluate incorrectly for non-ASCII strings. For example, "é𝄞" should be size 3 (1 + 2 surrogate pair) but was returning 6.

Testing done

• Verified against real DynamoDB: size("é𝄞") = 3 succeeds, size("é𝄞") = 6 fails
• Verified same behavior on ExtendDB after fix
• cargo test --workspace passes
• cargo clippy --all-targets -- -D warnings clean

Checklist

• I have read CONTRIBUTING.md
• All tests pass (cargo test --workspace)
• Code is formatted (cargo fmt --check)
• Clippy is clean (cargo clippy -- -W clippy::pedantic)
• I have added or updated tests for new functionality
• I have updated documentation if behavior changed
• Breaking changes are noted below (if any)

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache License 2.0 and I agree to the Developer Certificate of
Origin (DCO). See CONTRIBUTING.md for details.