TINKERPOP-3270 where(P) throws a descriptive error for non-String scope keys

[GumpacG]

Summary

where(P) expects a String scope key (e.g. where(eq('a'))). Given a non-String
predicate value like gt(5), WherePredicateStep cast it to String unconditionally,
throwing a raw ClassCastException at step construction. This change validates the
value first and throws a descriptive IllegalArgumentException pointing to is(P) for
value comparisons. Backward-compatible diagnostics fix; valid queries are unaffected.

Before vs After

Non-String (numeric) predicate value

__.values('v').where(gt(5))

• Before: ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String
• After: IllegalArgumentException: where(P) requires a String scope key but encountered Integer; use is(P) to compare values

Nested inside a parent where(Traversal)

g.V().hasLabel('X').where(__.values('v').where(gt(5)))

• Before: same ClassCastException, buried in a nested traversal
• After: same descriptive IllegalArgumentException pointing to is(P)

Correct usage (unchanged)

g.V().as('a').out().where(eq('a'))   // String scope key comparison
__.values('v').is(gt(5))             // value comparison via is(P)

• Before and after: works identically

Changes

• gremlin-core: runtime String validation in WherePredicateStep.getSelectKey(...)
• gremlin-test: negative tests added to the WhereTest suite (the exception is thrown at step construction, so they pass across all engines)
• CHANGELOG.asciidoc: entry under 3.7.7

Assisted-by: Kiro: Claude Opus 4.8

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.52%. Comparing base (4722890) to head (207b390).
⚠️ Report is 41 commits behind head on 3.7-dev.

Files with missing lines	Patch %	Lines
...emlin/process/traversal/step/filter/WhereTest.java	50.00%	8 Missing ⚠️
...cess/traversal/step/filter/WherePredicateStep.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             3.7-dev    #3559      +/-   ##
=============================================
+ Coverage      75.49%   75.52%   +0.02%     
- Complexity     13161    13183      +22     
=============================================
  Files           1092     1093       +1     
  Lines          67208    67249      +41     
  Branches        7391     7397       +6     
=============================================
+ Hits           50742    50792      +50     
  Misses         13837    13837              
+ Partials        2629     2620       -9     

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[xiazcy]

VOTE +1 pending comment resolution

[Cole-Greer]

VOTE +1, merging as CTR