fix(nano): fix nano_contracts_enabled on /version api

[glevco]

Motivation

The /version API was indirectly changed to return a string enum instead of a bool for the nano_contracts_enabled key, which made all clients believe the nano feature is active for all networks. This PR fixes this.

Acceptance Criteria

• Refactor is_nano_active logic into its own utils function.
• Change the nano_contracts_enabled key on the /version API to return a bool instead of an string enum.

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	feat/nano/enabled-api
Testbed	ubuntu-22.04

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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	1.91 m (+14.23%) Baseline: 1.67 m	1.51 m (78.79%)	2.01 m (95.19%)

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[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.81%. Comparing base (d6690ec) to head (773db89).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/nanocontracts/utils.py	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1387      +/-   ##
==========================================
- Coverage   85.82%   85.81%   -0.02%     
==========================================
  Files         429      429              
  Lines       32423    32433      +10     
  Branches     5057     5056       -1     
==========================================
+ Hits        27828    27831       +3     
- Misses       3583     3587       +4     
- Partials     1012     1015       +3     

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