feat: add validate_ratios and BASIS_POINTS_TOTAL for invoice ratio in…

[Keengfk]

Enforce ratio integrity at invoice creation time

Closes #[issue number]

Summary

Accepting split ratios that don't sum to exactly 10 000 basis points silently
causes under- or over-distribution of funds. This PR adds server-side
validation that rejects invalid ratios before any storage is written.

Changes

types.rs

• Added BASIS_POINTS_TOTAL = 10_000u32 constant as the single source of truth
  for the 100% threshold
• Added ratios: Vec field to InvoiceOptions — empty means "no ratio
  constraint, use amounts directly"

error.rs

• Added InvalidRatioSum = 33 — ratios provided but don't sum to 10 000
• Added EmptyRecipientList = 34 — ratios vec is non-null but empty

lib.rs

• Added validate_ratios(ratios: &Vec) -> Result<(), ContractError> private
  helper
• Called from create_invoice via env.panic_with_error() before any storage
  write

test.rs / fuzz/src/lib.rs

• Added ratios field to all default_options helpers (empty vec,
  backward-compatible)
• Added 7 unit tests covering all acceptance criteria

Acceptance criteria

┌───────────┬──────┐
│ Criterion │ Test │
├────────────────────────────────┼──────┤
│ Sum < 10 000 → InvalidRatioSum │ test_validate_ratios_under_sum_rejected │
├────────────────────────────────┼─────────────────────────────────────────┤
│ Sum > 10 000 → InvalidRatioSum │ test_validate_ratios_over_sum_rejected │
│ Sum > 10 000 → InvalidRatioSum │ test_validate_ratios_over_sum_rejected │
├──────────────────────────────────┼────────────────────────────────────────┤
│ Single entry = 10 000 → accepted │ test_validate_ratios_exact_sum_accepte │
│ │ d │
├──────────────────────────────────┼────────────────────────────────────────┤
│ Empty vec → EmptyRecipientList │ test_validate_ratios_empty_rejected │
├──────────────────────────────────┼────────────────────────────────────────┤
│ Validation before storage writes │ call site is before │
│ │ _create_invoice_inner │
│ │ _create_invoice_inner │
├─────────────────────────────────────┼─────────────────────────────────────┤
│ Valid ratios on create_invoice → │
│ succeeds │ cepted │
├─────────────────────────────────────┼─────────────────────────────────────┤
│ Invalid ratios on create_invoice → │
│ panics │ panics │
└─────────────────────────────────────┴─────────────────────────────────────┘

Migration note

InvoiceOptions gained a new ratios field. All existing callers that use struct
update syntax (..default_options(env)) are unaffected. Callers constructing
InvoiceOptions in full must add ratios: Vec::new(env) to preserve the existing
no-constraint behaviour.
closes #478

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