fix: ensure reentrancy guard is cleared on all exit paths in stake

[Kabi3si]

Summary

stake set REENTRANCY_GUARD but did not clear it on every early-exit path (particularly inside handle_cross_chain_stake which could panic after the guard was set). A stuck guard would permanently block future calls. Additionally, update_reward had no initialisation-check guard.

Changes

• contract/contracts/src/cross_chain.rs: refactor stake to use a clear closure that removes the guard before every panic! / early return. Move chain-validity checks into stake before delegating to handle_cross_chain_stake, so panics in validation clear the guard first.
• contract/contracts/src/storage.rs: add an early-return guard in update_reward when the contract is not yet initialised, preventing a spurious panic that would bypass the caller-side guard cleanup.

Testing

• A simulated token-callback re-entry into stake returns "reentrant call detected" without leaving the guard set.
• Normal stake flow clears the guard after committing user state.

Closes #512

[Xhristin3]

LGTM