Add unit tests for treasury address reads

[Olowodarey]

Adds focused unit tests for treasury address read operations, verifying predictable behavior across different states (unset, set, and persistent reads).

Changes

New Unit Tests (3 tests for treasury address)

1. test_get_treasury_address_returns_none_initially

   • Verifies get_treasury_address returns None when no address is configured
   • Tests the initial/unset state behavior

2. test_get_treasury_address_returns_set_address

   • Verifies get_treasury_address returns the configured address after setting
   • Tests the basic set-and-read flow

3. test_get_treasury_address_persists_across_reads

   • Verifies get_treasury_address returns consistent results across multiple reads
   • Tests read-only behavior and state persistence

Additional Test (1 test for creator fee config)

4. test_get_creator_fee_config_returns_defaults_for_unregistered
   • Verifies get_creator_fee_config returns default values for unregistered creators
   • Completes the not-registered creator read coverage

Test Characteristics

• Small and direct (each test focuses on one scenario)
• Clear assertions (expected values are explicit)
• Scoped to read paths (no state mutation beyond setup)
• Predictable return behavior (None, Some(address), consistent values)

Testing

All tests pass:

• ✅ 4 new unit tests (3 treasury + 1 fee config)
• ✅ 18 existing unit tests continue to pass
• ✅ 51 integration tests continue to pass
• ✅ Total: 73 tests passing

Acceptance Criteria

• ✅ Treasury address read behavior is covered by tests
• ✅ Assertions are clear and specific (None, Some(address), equality checks)
• ✅ Tests stay scoped to the read path (read-only operations)

Quality Checks

• ✅ All tests pass (cargo test)
• ✅ Code formatted (cargo fmt)
• ✅ No clippy warnings (cargo clippy)
• ✅ Test snapshots generated

Design Notes

These unit tests complement the existing integration tests in tests/treasury_address.rs by providing focused, isolated coverage at the unit level. They verify:

• Initial state returns None
• Set address is retrievable
• Multiple reads return consistent results
• No state mutation occurs during reads

The tests follow the same pattern as other read method tests, ensuring consistency across the test suite.

closes #38
closes #39

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