feat: add regression tests and fix sweep self-transfer in maintenance-pool

[anuoluwaponiorimi]

Fixes and adds regression coverage for four reported gaps in maintenance-pool.

#481 — sweep accepts the contract's own address as recipient (regression on #329)
Added a guard at the top of sweep that returns Error::InvalidTreasury when recipient == env.current_contract_address(), mirroring the identical guard already present in initialize. Added test_sweep_rejects_contract_as_recipient.

#467 — set_admin has no test (regression on #249)
Added three tests: test_set_admin_rotates_admin (happy path), test_set_admin_rejects_non_admin_caller, and test_set_admin_requires_new_admin_auth.

#482 — withdraw accepts zero/negative amount (regression on #44)
Added test_withdraw_rejects_zero_amount and test_withdraw_rejects_negative_amount; both assert Error::InvalidAmount.

#478 — initialize accepting treasury == contract address (regression on #341)
Added test_initialize_rejects_treasury_as_contract_address and test_initialize_rejects_max_fee_bps.

Closes #481
Closes #467
Closes #482
Closes #478

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