[Maikol]

Removes packages/contracts and packages/contracts-test by moving every live contract into the package that owns its domain (graph-core, graph-token, subgraph-discovery, horizon, issuance) with unchanged bytecode, porting their tests to Foundry, and deleting dead code. Each phase lands here as its own reviewed PR, following the design and plans in #1372.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (3c7ca93) to head (1ded9c9).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1373    +/-   ##
========================================
  Coverage   91.47%   91.47%            
========================================
  Files          81       81            
  Lines        5326     5326            
  Branches      973     1132   +159     
========================================
  Hits         4872     4872            
  Misses        433      433            
  Partials       21       21            

Flag	Coverage Δ
unittests	91.47% <ø> (ø)

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