Settlement: developer balance upsert behavior tests

[greatest0fallt1me]

Description

Multiple credits accumulate; defaults behave as documented for missing keys.

Requirements and context

• Guard against accidental zero-entry storage if undesired.
• Add regression tests.

Suggested execution

• Fork the repo and create a branch: git checkout -b test/settlement-developer-map-upsert
• Implement changes in the Callora-Contracts Soroban workspace (contracts/*/src/).
• Run cargo fmt, cargo clippy --all-targets --all-features -- -D warnings, and cargo test from the workspace root.
• For WASM release builds: cargo build --target wasm32-unknown-unknown --release -p callora-settlement (adjust -p if the crate name differs).

Deliverables

• contracts/settlement/src/test.rs.

Test and commit

• Run tests and ./scripts/coverage.sh (or cargo tarpaulin per tarpaulin.toml).
• Cover edge cases; include summarized test output and brief security notes in the PR description.

Example commit message

test(settlement): developer balance upsert behavior

Guidelines

• Minimum 95% line coverage for touched crates (workspace policy).
• Clear documentation (Rust /// on public items where applicable; repo markdown as needed).
• Timeframe: 96 hours from assignment unless agreed otherwise.

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[jaymhorsh]

@jaymhorsh has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would like to work on this issue, I have ETc of 2-3hrs.

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