fix: invoice funded_amount field not updated on partial funding — shows zero

[sanmipaul]

Bug

`contracts/invoice/src/lib.rs` or `contracts/pool/src/lib.rs` — if invoices support partial funding (multiple lenders funding portions of an invoice), the `funded_amount` field in the `Invoice` struct may not be updated correctly after each partial funding event. The field may remain at zero or only reflect the last funding amount rather than the cumulative total.

Impact

• UI shows incorrectly funded amount on invoice detail page
• Outstanding balance calculations based on `funded_amount` are wrong
• Collateral ratio checks use wrong reference amount
• Events emitting `funded_amount` contain incorrect data

Fix

Update `funded_amount` using checked addition on every funding:

```rust
pub fn fund_invoice(env: Env, invoice_id: u64, amount: i128) {
let mut invoice = get_invoice(&env, invoice_id)?;
// ... checks ...
invoice.funded_amount = invoice.funded_amount
.checked_add(amount)
.ok_or(PoolError::AmountOverflow)?;
save_invoice(&env, invoice_id, &invoice);
// ... interactions ...
}
```

Acceptance Criteria

• `funded_amount` accumulates correctly across multiple partial fundings
• Unit test: two partial fundings → `funded_amount == sum of both`
• Unit test: full single funding → `funded_amount == invoice amount`
• No use of `saturating_add` for `funded_amount` accumulation
• `get_invoice()` returns correct `funded_amount` after each funding

References

• `contracts/invoice/src/lib.rs` — `Invoice` struct, `funded_amount` field
• `contracts/pool/src/lib.rs` — `fund_invoice()`

[jd5073356-max]

I can resolve the partial funding accumulation bug by updating the fund_invoice logic within contracts/pool/src/lib.rs to use checked_add on the Invoice struct's funded_amount field. I will ensure that each partial funding event cumulatively increments the state instead of overwriting it or remaining at zero, handling the PoolError::AmountOverflow safely. Finally, I'll add unit tests in the test suite to validate that multiple partial fundings correctly sum up, a single full funding matches the invoice amount, and get_invoice() returns the precise cumulative total. I can have this PR submitted with passing tests within 1 day.