Added Java driver tests for coverage

[GumpacG]

Increase gremlin-driver test coverage

Adds targeted unit tests for gremlin-driver that exercise previously-untested library
code: the HTTP request/response codecs, remote traversal and transaction handling,
round-robin load balancing, Auth factory dispatch, connection-pool lifecycle, stream
buffers, and Cluster/Host configuration. Tests only, no production code changes.

Coverage (gremlin-driver aggregate: unit + integration)

Metric	Baseline (master)	With added tests
Instructions	76.19%	82.68%
Branch	64.01%	72.28%

~83 new driver unit tests. The full gremlin-driver + gremlin-server unit and
integration suites pass.

Coverage reports

Baseline (master: 2e56ccc):

With added tests:

Assisted-by: Kiro: Claude Opus 4.8

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.91%. Comparing base (a28cd1f) to head (9e2010a).
⚠️ Report is 377 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3575      +/-   ##
============================================
+ Coverage     76.35%   76.91%   +0.55%     
- Complexity    13424    14416     +992     
============================================
  Files          1012     1036      +24     
  Lines         60341    64608    +4267     
  Branches       7075     7661     +586     
============================================
+ Hits          46076    49696    +3620     
- Misses        11548    11846     +298     
- Partials       2717     3066     +349     

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