Added coverage for gremlin-console and gremlin-language

[GumpacG]

Test coverage improvements

Two related but independent changes that improve test coverage in TinkerPop, each self-contained:

1. Grammar parser coverage - raises GremlinParser coverage in gremlin-language.
2. gremlin-console coverage - adds unit + end-to-end (pexpect) tests for the console.

Both changes also remove dead code found while raising coverage. Removals are listed first, followed by a consolidated coverage summary with the before/after numbers and an explanation of why coverage doesn't go higher.

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Dead code removed

gremlin-language / gremlin-core (grammar)

• Removed the orphaned booleanArgument, dateArgument, and
  nullableGenericLiteralMap grammar rules (unreachable from queryList after
  their references were dropped in earlier changes), plus the coupled
  gremlin-core base-visitor stubs, ArgumentVisitor/TranslateVisitor
  methods, and obsolete ArgumentVisitorTest cases. Regenerated the gremlin-js
  parser and removed its dead visitBooleanArgument.

gremlin-console

• Console.groovy: unused private writeTraverserToErrorLines.
• PluggedIn.groovy: never-instantiated GroovyGremlinShellEnvironment inner class.
• Deleted the unused MockGroovyGremlinShellEnvironment test helper.

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Coverage

gremlin-console

Caveat: the standard build runs the pexpect tests in Docker without the JaCoCo agent, so CI's JaCoCo run won't reflect the pexpect-driven portion; the 83% figure was obtained by attaching the agent to the console JVM the pexpect tests spawn.

before (~50%):

after (~83%):

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gremlin-language (grammar / GremlinParser)

• gremlin-language grammar package: 56.98% -> 60.25% (instructions).
• GremlinParser main rule class: 76.2% -> 81.2% instructions, 62.8% -> 69.2% branches.
• Aggregate GremlinParser (gremlin-language + full gremlin-core suite): 57.5% -> 61.3%.
• ~33 gremlin-core grammar visitor methods moved from 0% to covered.

before (~57%):

before (~60%):

GremlinParser before:

GremlinParser after:

Why coverage doesn't go higher

gremlin-console

• InstallCommand's successful dependency-download branch (the real Grape.grab + post-download plugin load) needs Grape + network, so it can't run in the offline test environment.
• RemoteGremlinPlugin and remote query submission only do real work against a live Gremlin Server. Covering them from gremlin-console would require standing up an embedded server or adding a test dependency on gremlin-server/gremlin-driver - out of scope for this module, and already covered by those modules' own integration tests - so those paths are intentionally left unexercised here.
• The StackOverflowError hint branch in handleError stays uncovered: triggering a real StackOverflowError on demand is flaky and depends on the JVM -Xss stack size, so a test for that single hint message would be brittle. (The other interactive error branches - the "Display stack trace? [yN]" y/n answers, the Failure formatter, and the non-interactive -e script-error exit - are covered by the pexpect suite.)

gremlin-language (GremlinParser)

GremlinParser is ANTLR-generated, so the remaining uncovered code is either
unreachable by valid input or only coverable by tests that assert nothing
meaningful. These additions already cover essentially all the reachable,
behaviour-bearing productions:

• ANTLR error-recovery paths - every generated rule method has a
  catch (RecognitionException) recovery block. The grammar test harness installs
  BailErrorStrategy, which aborts on the first error, so recovery is never
  entered; reaching those blocks would mean re-parsing malformed input under a
  non-bail error strategy the harness deliberately doesn't use. This is the single
  largest uncovered bucket.
• Grammar-shadowed alternatives - some labeled alternatives are always
  pre-empted by an earlier alternative that matches the same input (e.g.
  by(Order)/by(T), and the nested-traversal has/property/hasValue
  forms, which the *_Object alternatives subsume). ANTLR never builds those
  context nodes, so no input can cover them; making them reachable would require
  reordering the grammar (out of scope).
• Unused generated accessors - *Context accessor methods such as
  KeywordContext and TraversalGTypeContext token getters are never called
  (consumers read ctx.getText()), and the generated no-op listener is unused.
  The only way to cover them is to invoke the generated getters directly, which
  asserts nothing about behaviour, so such a test would only game the metric.
• Generated infrastructure - getSerializedATN, getATN, sempred, etc. are
  ANTLR boilerplate; invoking them in a test would colour the lines without
  verifying anything, so covering them would only inflate the number.