test: cover milestone and maintenance edge cases

[prissca]

Summary

• mark escrow, milestones, and maintenance-pool public Error enums as non-exhaustive
• add milestones tests for fee_bps > 10000 and zero-fee full payout
• add maintenance-pool coverage for get_pool and withdraw on missing pool IDs

Tests

• cargo fmt
• git diff --check
• cargo test -p mergefi-milestones -p mergefi-maintenance-pool (blocked locally: Windows MSVC linker link.exe is not installed)

Closes #81
Closes #82
Closes #83
Closes #95

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