Add pox-4.clar to Clarinet.toml

[friedger]

Description

This PR adds pox-4.clar to Clarinet.toml so that developers can use clarinet check when making changes to pox-4 contract.

Since Clarinet 2, the different constants for mainnet and testnet can be handled in a single file (pox-4.clar without pox-mainnet or pox-testnet) using is-in-mainnet.

Applicable issues

No issue. Just chore for developer experience

Additional info (benefits, drawbacks, caveats)

This change was initially proposed and already reviewed in #4094.

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3094df2) 83.34% compared to head (eaa8363) 83.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4194      +/-   ##
==========================================
- Coverage   83.34%   83.31%   -0.03%     
==========================================
  Files         432      432              
  Lines      306557   306556       -1     
==========================================
- Hits       255490   255408      -82     
- Misses      51067    51148      +81     

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