Add fuzz tests for `fund_invoice`

**Description:**

The fund_invoice function needs property-based fuzz testing.

**Why it matters:** Fuzz testing finds edge cases that unit tests miss.

**Acceptance Criteria:**
- [ ] Property test that `fund_invoice` never panics with random inputs
- [ ] Test with random amounts, tokens, and funder addresses

**Relevant Files:** `contracts/fuzz/src/lib.rs`

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