Test `record_invoice_created` increments the invoice counter by one

[Kingsman-99]

Description

record_invoice_created in contracts/split/src/stats.rs increments a persistent counter. There is no focused unit test checking that a single call advances the counter from 0 to 1 and emits a StatsUpdated event. This test would catch accidental no-ops or double-increment bugs.

Acceptance Criteria

• Add a test named record_invoice_created_increments_counter in contracts/split/src/stats.rs
• Call get_stats before and after record_invoice_created
• Assert the invoice count changed from 0 to 1; volume and recipients_paid unchanged
• cargo test passes

Context

• Target file: contracts/split/src/stats.rs
• Use soroban_sdk::testutils::Env with allow_non_root_auth

[quickweb-stack]

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[maugauwi-hash]

@maugauwi-hash has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer, I’d be glad to take on this issue. I’ve worked on similar tasks before and have the experience needed to handle it

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[grantfox-oss]

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@maugauwi-hash's PR #645 was approved and merged by @Kingsman-99.

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