Fix(finstripe): reject zero-and-negative amount transfers with pre-DB guard

[Jean-Regis-M]

Summary

Fixes #334

Adds amount <= 0 validation to create_transfer, blocking zero-amount
transactions before any DB write or transfer ID generation occurs.

Problem

create_transfer(amount=0.0, ...) succeeded, creating a completed
transaction record — polluting audit logs with financially meaningless
entries and allowing invoice/vendor ID probing at zero cost.

Root Cause

No amount validation existed in create_transfer. The function
called _generate_transfer_id() and repo.create_transaction()
unconditionally, regardless of amount value.

Solution

Insert a two-line guard at function entry:

if amount <= 0:
    return {"error": f"Invalid amount: {amount}. Amount must be greater than zero."}

Returns before any side-effectful call is made.

Impact

• No breaking changes
• Minimal diff (2 lines added)
• Deterministic behavior
• Zero regression risk to valid-amount paths

Testing

pytest tests/unit/mcp/test_finstripe.py::TestFloatEdgeCases::test_mcp_float_005_zero_amount_raises -v
pytest tests/unit/mcp/test_finstripe.py::TestFloatEdgeCases::test_mcp_float_001_max_payment_boundary_accepted -v