fix(nano): Stop sharing NanoRNG object between contracts

[msbrogli]

Motivation

The Nano runner previously returned the same NanoRNG instance to all contracts, allowing potential exploits where an attacker could override methods in one contract to manipulate random number generation in others. To mitigate this, several layers of protection have been introduced:

• Class-level Protection: Method overriding is prevented using __setattr__() in a metaclass, ensuring the class definition remains immutable.
• Instance-level Protection (Setattr): Direct overrides via __setattr__() are blocked on individual instances.
• Instance-level Protection (Slots): Methods are made read-only through the use of __slots__, restricting attribute modifications.
• Instance Isolation: Unique NanoRNG instances are now provided to each contract, eliminating sharing.
• Class Isolation: Instances are created via a dynamic shell subclass, ensuring that even the NanoRNG class itself is not shared across contracts.

These measures collectively enhance security by isolating RNG behavior and preventing unauthorized modifications. These measures can also be applied to other classes and instances accessible by contracts.

Original discussion: https://github.com/HathorNetwork/nano-hathor-core/pull/168#discussion_r2025167397

Acceptance Criteria

1. Add a test that exploits the vulnerability.
2. Stop sharing NanoRNG objects between contracts.
3. Each contract will have a single NanoRNG object which will be shared in case of multiple access by that contract.
4. Add slots to NanoRNG.
5. Prevent assigning methods in NanoRNG instances.
6. Prevent assigning methods in NanoRNG classes.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[github-actions]

Bencher Report

Branch	fix/nano-shared-rng
Testbed	ubuntu-22.04

Click to view all benchmark results

Benchmark	Latency	Benchmark Result minutes (m) (Result Δ%)	Lower Boundary minutes (m) (Limit %)	Upper Boundary minutes (m) (Limit %)
sync-v2 (up to 20000 blocks)	📈 view plot 🚷 view threshold	1.66 m (+1.54%) Baseline: 1.64 m	1.47 m (88.63%)	1.80 m (92.31%)

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[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.66%. Comparing base (5c883a7) to head (1927f37).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/nanocontracts/runner/runner.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   85.65%   85.66%           
=======================================
  Files         424      424           
  Lines       32095    32120   +25     
  Branches     4994     4997    +3     
=======================================
+ Hits        27492    27516   +24     
- Misses       3603     3604    +1     
  Partials     1000     1000           

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