Add zero-supply sell underflow test

[kitWarse]

Summary

• Adds a targeted sell-path regression test for the impossible state where a registered creator has zero total supply while the seller balance is nonzero.
• The test confirms sell_key returns ContractError::SellUnderflow instead of panicking on arithmetic underflow, and verifies the failed call leaves observable state unchanged.

Issue

• Closes Add unit tests for sell revert when creator has zero total supply #370

Changes

• Updated creator-keys/tests/sell_underflow.rs with a distinct zero-supply creator sell test.
• Sets up the impossible state by writing only the seller balance key under the test contract context while leaving creator supply at zero.
• Uses capture_snapshot before and after the failed sell to assert supply, holder count, and seller key balance are unchanged.

Validation

• Ran cargo fmt.
• Ran git diff --check.
• Ran cargo test -p creator-keys test_sell_registered_zero_supply_creator_returns_sell_underflow_without_state_change, but local compilation could not proceed because this Windows environment is missing the MSVC linker link.exe before repo code compiled.

Notes

• The local test failure appears environment-related: Rust MSVC targets require Visual Studio Build Tools with the C++ linker installed.
• Fork relationship verified: kitWarse/accesslayer-contracts is a fork of accesslayerorg/accesslayer-contracts.

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