Implement contract security guards and advanced dependencies

[Dopezapha]

Closes #472
Closes #458

Summary

This PR implements two contract-level improvements for SoroTask:

1. Reentrancy protection and security guards for state-mutating flows.
2. Advanced task dependency rules based on explicit execution outcomes.

Changes

#472 Contract Security Guards

• Added a contract-wide reentrancy lock backed by instance storage.
• Protected state-mutating functions from nested mutation during external calls.
• Guarded execution paths that call resolver contracts, target contracts, and token contracts.
• Added a dedicated ReentrantCall contract error.
• Preserved existing authorization checks using require_auth for creators, keepers, and gas depositors.
• Added a regression test that proves a target contract cannot reenter and pause another task during execute.

#458 Advanced Task Dependencies

• Added execution status tracking for each task.
• Added support for dependency rules that can require:
  • successful execution
  • skipped execution
  • any completed execution
• Added completed_at and run_count tracking for dependency evaluation.
• Added add_dependency_with_rule, get_task_status, get_dependency_rules, and is_dependency_satisfied.
• Kept the existing add_dependency API compatible by mapping it to a default success-based dependency rule.
• Added bounded dependency fan-in and dependency depth checks to keep graph validation predictable.
• Added focused tests for skipped-outcome dependencies and minimum completion timestamp rules.

Security Notes

• Reentrancy-sensitive external calls are protected by the mutation lock.
• Read-only contract calls remain available while the guard protects writes.
• If a guarded transaction fails, Soroban rollback reverts the lock state with the rest of the transaction.
• Dependency graph writes are bounded to avoid unbounded traversal cost.

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