Replace the deprecated Pydantic v1 @validator('amount', pre=True,
always=True) with a v2 @model_validator(mode='after'). The model
validator always runs and can see both amount and send_all, preserving
the cross-field rule (and the always=True semantics that reject the
empty/default case). Removes the last Pydantic v1 deprecation warning.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
