test(cost): add unit test for max_daily_cents=0 unlimited budget

[bug-ops]

Summary

• Adds check_budget_unlimited_when_max_daily_cents_is_zero to crates/zeph-core/src/cost.rs
• Covers the branch if self.max_daily_cents > 0.0 && ... — confirms that max_daily_cents = 0.0 means unlimited regardless of accumulated spend
• Updates CHANGELOG.md [Unreleased] Testing section

Test plan

• cargo nextest run -p zeph-core -E 'test(check_budget_unlimited)' — passes
• cargo +nightly fmt --check — OK
• cargo clippy --workspace -- -D warnings — OK
• cargo nextest run --workspace --lib --bins — 5915 passed

Closes #2110