[Mark Birger (kusha)]

Summary

Use serde_json::to_string in regorus_rvm_get_host_await_argument to avoid pretty-printing overhead at the Rust-to-C# boundary. The change is local to this accessor: public Value::to_json_str(), unrelated FFI accessors, error propagation, and memory-budget/unwind guards remain unchanged.

Add an exported-FFI regression checking exact compact output for a single string (["abc"]), escaped characters including NUL, meaningful whitespace inside strings, and a nested object containing an array, numbers, a boolean, null, and an escaped string. Deserialize the actual output to verify argument-array shape and value preservation.

Validation

• Confirmed the original regression fails against pretty-printed output before the serialization change.
• All 34 RVM FFI tests passed using cargo test --manifest-path bindings/ffi/Cargo.toml --no-default-features --features std,rvm,regorus/arc,contention_checks,allocator-memory-limits rvm:: -- --test-threads=1.
• The expanded regression including the nested-object case passed after the review update.
• Formatting and diff checks passed; the complete diff was inspected.
• Default-feature FFI testing is blocked by pre-existing deprecated_in_future errors for core::char::from_digit in Azure Policy code.
• The pre-commit hook passed the root all-targets build and formatting, then failed on the same Azure Policy errors and unrelated existing Clippy warnings. Git hooks were bypassed for commit and push with explicit user approval.