refactor(storage): consolidate admin_set_yield_rate ledger footprint

[Awosdot]

Summary

• Introduces a single YieldConfig { pub yield_rate: u32 } struct stored under DataKey::YieldConfig, replacing the standalone DataKey::YieldRateBps key in both contracts/milestone-escrow/src/lib.rs and contracts/reports/src/lib.rs.
• Updates admin_set_yield_rate and the get_yield_info getter in both files to read/write through the consolidated struct, reducing the ledger footprint of yield-rate related writes.

Test plan

• cargo test passes (449 tests) for the milestone-escrow workspace crate.
• Verified contracts/reports/src/lib.rs compiles and its test suite runs cleanly against the change (one pre-existing, unrelated failure — test_multisig_transfer_admin_high_precision_split_preserves_every_unit — reproduces identically on main before this change, so it is not a regression).

Closes #407

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[Awosdot]

Please check and merge