Add ReentrancyGuard::scope_stack debug introspection helper

Description

reentrancy_guard.rs keeps a per-entrypoint scope but does not expose its current depth for tests. Add a #[cfg(test)] pub fn current_scope_stack() helper returning the active scope chain so integration tests can assert correct nesting and detect accidental cross-scope leakage.

Requirements and Context

• Helper compiled only under #[cfg(test)]
• Returns Vec<Symbol> of active scopes
• Used by at least two existing tests
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b task/reentrancy-scope-introspection
   

2. Implement changes
   • contracts/predictify-hybrid/src/reentrancy_guard.rs
   • Update an existing test in tests/
3. Test and commit
   • cargo test -p predictify-hybrid reentrancy
   • Cover edge cases
   • Include test output and notes in the PR

Example commit message

test: expose reentrancy guard scope stack for assertions

Acceptance Criteria

• Helper is gated to cfg(test)
• Two pre-existing tests adopt the helper
• No public surface change for production builds

Guidelines

• NatSpec-style /// doc comments where relevant
• Clear documentation and inline comments
• Timeframe: 96 hours