test: airdrop reverts when recipient list exceeds 50 addresses

[OpensrcLord]

Summary

Closes #537.

The airdrop entrypoint did not exist yet (tracked in #523), so this PR first adds airdrop_keys and then the boundary tests #537 asks for.

airdrop_keys (prerequisite, per #523 spec)

• Creator-only: mints keys to a list of AirdropEntry { address, amount } recipients; non-creator callers revert with Unauthorized
• Creator pays the bonding curve cost for every key plus the protocol fee on the total; creator fee is waived
• Supply increases by the total airdropped amount, moving the curve exactly as individual buys would
• Max 50 recipients per call (MAX_AIRDROP_RECIPIENTS); larger lists revert with the new AirdropRecipientLimitExceeded error (appended at the end of the enum, ABI-safe)
• The whole airdrop is priced and validated before any storage write, so a failed call mints nothing
• Emits KeysAirdropped { creator_id, total_keys, total_cost, recipient_count, ledger } and returns AirdropSummary { total_keys, total_cost, recipient_count }

Tests (creator-keys/tests/airdrop_recipient_limit.rs)

• 51 recipients reverts with clear error — asserts AirdropRecipientLimitExceeded
• 50 recipients succeeds — summary fields, supply, holder count, and every recipient balance verified
• No state change after failed airdrop — seeds non-trivial state first, then asserts supply, holder count, protocol fee balance, and all 51 recipient balances are untouched after the revert

Build fix

ethnum 1.5.2 fails to compile on current stable rustc (1.97); bumped to 1.5.3 in Cargo.lock (no other dependency changes).

Verification

• cargo test --workspace — all 120 test targets pass
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --check — clean