contracts(multisig): validate amount > 0 and to != from in submit_transaction

[benzy018]

Summary

Fixes the two validation gaps identified in issue #130.

Changes in contracts/multisig/src/lib.rs:

• Added SelfTransfer = 14 variant to MultiSigError enum
• submit_transaction now rejects amount <= 0 with InvalidAmount (#6) before storing any state
• submit_transaction now rejects caller == to with SelfTransfer (#14) before storing any state

Why

• Storing transactions with amount = 0 or negative amounts silently inflates on-chain pending-tx storage budget with useless entries.
• Self-transfers are a no-op griefing vector that wastes signer approvals and storage.

Tests Added

Test	Expects
test_submit_transaction_rejects_zero_amount	Error(Contract, #6)
test_submit_transaction_rejects_negative_amount	Error(Contract, #6)
test_submit_transaction_rejects_self_transfer	Error(Contract, #14)

All three tests follow the exact same pattern as the existing test_cannot_initialize_twice test using Soroban's #[should_panic(expected = "Error(Contract, #N)")] convention.

Acceptance Criteria

• Amounts ≤ 0 are rejected
• Self-transfers (to == caller) are rejected
• New tests cover both rejection cases

Closes #130