Add sell holder count unit tests 675

[somotochukwu-dev]

Summary

Adds a unit test for the sell function correctly decrementing the holder count when a wallet sells all its keys.

Changes

• creator-keys/tests/sell_key.rs: Added test_multi_key_full_exit_decrements_holder_count_only_on_last_sell — a wallet holding 3 keys sells 2 (holder count unchanged), then sells the last key (holder count decrements to 0). This fills the gap where no existing test verified holder count only decrements on the final sale when a wallet holds more than one key.

Acceptance criteria coverage

The following scenarios were already covered by existing tests and remain passing:

Criterion	Existing test
Selling all keys decrements holder count by 1	test_sell_key_removes_holder_when_last_key_is_sold
Selling partial keys does not change holder count	test_sell_key_preserves_holder_count_when_seller_still_has_keys
Two wallets selling all keys decrements count by 2	test_holder_count_reflects_mixed_trade_correctly in supply_invariants.rs
Holder count reaches 0 when the last holder sells all keys	Covered by full-exit tests above
State correct after each scenario	All tests assert state after each operation

The new test covers the previously untested scenario of a multi-key wallet fully exiting.

Closes #675

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