test: prevent currency removal from orphaning active escrows

[brentthomas248]

Closes #1709

Summary:

• reject currency whitelist removals and set replacements that would remove a currency with a nonzero held-escrow reserve
• add a regression proving removal is rejected while escrow is held and succeeds after refund releases the reserve
• carry current-toolchain CI/test repairs needed for upstream main to compile and run under the current Soroban/Rust toolchain

Verification:

• cargo test -p quicklendx-contracts remove_currency_rejects_active_held_escrow_and_allows_after_refund -- --nocapture
• cargo test --verbose
• cargo clippy -p quicklendx-contracts --all-targets -- -D warnings
• quicklendx-contracts/scripts/check-wasm-size.sh
• cargo llvm-cov --lib --lcov --output-path coverage/lcov.info && ADMIN_COVERAGE_MIN=40 bash scripts/check-admin-coverage.sh coverage/lcov.info
• git diff --check

Note:

• cargo fmt --check is blocked by an existing malformed legacy test module: quicklendx-contracts/src/test_max_invoices_per_business.rs has an unclosed delimiter. Touched files were formatted directly with rustfmt --config skip_children=true where needed.