TINKERPOP-2063 Add Tree support to the GLVs (Python, .NET, Go, JavaScript)

[GumpacG]

Summary

This PR brings a matching native Tree type to all Gremlin Language Variants so that a server-produced
tree() result deserializes into a first-class object with the same read/navigation
API instead of an untyped map (or, in JavaScript, a serializer stub that threw).

The work is deserialization + read-navigation focused: trees are computed on the
server and streamed to the client, so each GLV gains (1) a native Tree type and
(2) a GraphBinary 0x2b (de)serializer. A symmetric serialize path is included for
round-trip/test symmetry.

What changed per variant

• gremlin-python: Tree class in structure/graph.py; TreeIO in graphbinaryV4.py
  (auto-registered for 0x2b); removed the @StepTree feature-test skip.
• gremlin-dotnet: Structure/Tree.cs and GraphBinary4/Types/TreeSerializer.cs;
  registered DataType.Tree; implemented the Gherkin tree-structure assertion; removed
  the @StepTree skip.
• gremlin-go: driver/tree.go plus treeType (0x2b), readTree/treeWriter, and
  registration; removed the ~@StepTree cucumber tag.
• gremlin-javascript: Tree class in structure/graph.ts; TreeSerializer.js
  replacing the previous stub; exported Tree; removed the named tree ignores.

Testing

• Per-GLV unit tests covering the full read API, structural equality, GraphBinary
  round-trips (nested, empty, null-key), duplicate-sibling-key collapse, element-key
  equality (by id and concrete type), childAt on a missing key, addTree null
  rejection, splitParents edge cases, and getLeafTrees.
• Per-GLV integration tests run g.V(1).out().out().tree() against the modern graph
  and navigate the native result.
• The shared Gherkin tree() feature scenarios were un-skipped in each GLV and assert
  tree structure through the read API.

Notes

• Scope is the GLVs; the Java core change shipped in Make Tree no longer extend HashMap #3448.
• Tree results travel server -> client; the serialize path exists for symmetry and
  round-trip tests rather than a common workflow.

https://issues.apache.org/jira/browse/TINKERPOP-2063

Assisted-by: Kiro: Claude Opus 4.8

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.18%. Comparing base (a28cd1f) to head (315f4c8).
⚠️ Report is 167 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3495      +/-   ##
============================================
- Coverage     76.35%   76.18%   -0.18%     
- Complexity    13424    13864     +440     
============================================
  Files          1012     1031      +19     
  Lines         60341    62918    +2577     
  Branches       7075     7338     +263     
============================================
+ Hits          46076    47931    +1855     
- Misses        11548    12014     +466     
- Partials       2717     2973     +256     

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