test(fee): add recipient validation tests and error stability checks

[Osuochasam]

Overview
This PR introduces explicit test coverage for the validate_recipient logic within the fee contract. It ensures that withdrawal transactions sent to the contract itself or to the hardcoded ZERO_CONTRACT_ADDRESS correctly fail with an InvalidRecipient error. Most importantly, it proves that the hardcoded zero address parses safely via Address::from_str without trapping, which would otherwise brick fee payouts. It also locks in the associated error code to prevent undocumented API breaks.

Key Changes

Recipient Validation Tests (contracts/fee/...):

Added a test verifying that attempting to withdraw to the contract's own address correctly returns InvalidRecipient.

Added a test verifying that withdraw to ZERO_CONTRACT_ADDRESS returns InvalidRecipient, effectively proving the constant parses successfully.

Added a success case test verifying that withdrawals to a standard, valid address process correctly.

Error Code Stability (contracts/fee/tests/err_stab.rs):

Appended InvalidRecipient = 8 to the error stability assertions to ensure the error code does not silently drift in future refactors.

closes #1191

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