test: add zero duration pool coverage (#496)

[edehvictor]

Closes #496

Changes

• add a unit test in test.rs covering pool creation with zero duration
• verify that create_pool fails when end_time == current ledger timestamp
• keep the new test alongside the existing pool-duration boundary tests for clarity

Testing

• cargo fmt
• cargo test test_create_pool_rejects_zero_duration -- --nocapture (blocked locally: missing MSVC linker link.exe on this machine)

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