Subgraph support for GLVs

[GumpacG]

Description

Brings gremlin-javascript, gremlin-dotnet, and gremlin-go to feature parity with gremlin-python for the GraphBinary 4.0 Graph type (0x10), so that g.X().subgraph('sg').cap('sg').next() from any of the four GLVs returns a usable Graph data container instead of throwing an unknown-type error.

Added unit tests and feature tests.

Assisted-by: Devin: Claude Opus 4.7

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.25%. Comparing base (cfd6889) to head (6986189).
⚠️ Report is 1052 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3428      +/-   ##
============================================
- Coverage     77.87%   76.25%   -1.62%     
+ Complexity    13578    13434     -144     
============================================
  Files          1015     1013       -2     
  Lines         59308    60593    +1285     
  Branches       6835     7091     +256     
============================================
+ Hits          46184    46205      +21     
- Misses        10817    11614     +797     
- Partials       2307     2774     +467     

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