fix: reject invalid data sizes

[JSap0914]

Fixes #449.

Summary

• split data-size suffixes on UTF-8 character boundaries instead of byte offsets
• use checked arithmetic when applying B/K/M/G/T multipliers
• add regression tests for multibyte suffixes and overflowing values

Validation

• cargo test test_rejects -- --nocapture
• cargo test
• cargo fmt --all -- --check
• cargo clippy -- -D warnings
• LSP diagnostics for src/models/data_size.rs: no diagnostics found
• cargo run -- create demo --disk '10€' -i ubuntu:noble
• cargo run -- create demo --disk '18446744073709551615T' -i ubuntu:noble

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[rogkne]

Thanks @JSap0914. The change looks good!
Please add a test case for an empty string (DataSize::from_str("")) and resolve the CI failure.

[rogkne]

@JSap0914 Please address the requested changes. If the requested changes aren't addressed, we'll need to close this PR.