fix(typecheck): hint at String.repeat for string multiplication

[PandaHUN777]

Summary

• Suggest String.repeat count string when * receives a string operand.
• Preserve the existing String.concat guidance for + and generic numeric errors elsewhere.
• Add coverage for both operand orders and unrelated numeric mismatches.

Closes #133

Validation

• cargo fmt --check
• cargo clippy --all-targets
• cargo test
• cargo build