fix: harden transaction numeric validation

[cr-shah]

Finding

Malformed numeric values in PaySim-compatible frames could escape the documented validation boundary as raw pandas exceptions. Fractional or out-of-range time steps were also accepted even though history features assume integer hours.

Fix

• normalize numeric amount, step, and isFraud fields
• reject non-finite, fractional, out-of-range, and non-binary values with DataValidationError
• add regression coverage for malformed values and numeric CSV strings

Verification

• tracked Python files: ruff check passed
• full suite: 63 tests passed
• focused validation suite: 20 tests passed
• regenerated demo JSON, CSV, and PNG outputs match checked-in artifacts byte-for-byte