Enable RuboCop Lint/UselessAssignment

[kpumuk]

Enable Lint/UselessAssignment and remove assignments whose values are never read across the Ruby library, benchmarks, specs, integration client, and tutorial.

The Ruby generator now binds decoded service arguments and results only when generated methods consume them. Focused functional coverage verifies zero-argument processor methods and void client receivers. The Ruby cross-test client also exercises its escape/junk string and multilingual string separately, fixing an overwritten test input exposed by the cop.

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[Copilot]

Pull request overview

Enables Lint/UselessAssignment for the Ruby codebase and updates Ruby sources/tests plus the Ruby code generator to avoid assigning values that are never read, while keeping behavior intact.

Changes:

• Enabled Lint/UselessAssignment in lib/rb/.rubocop.yml and removed/adjusted unused assignments across Ruby library code, specs, benchmarks, integration tests, and tutorial code.
• Updated the Ruby generator to only bind decoded args/results when the generated methods actually consume them (void/no-arg cases avoid unused locals).
• Added/adjusted functional coverage for the Ruby generator and strengthened the Ruby integration string test to exercise both escaped and unicode inputs.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tutorial/rb/RubyClient.rb	Removes an unused assignment in the divide-by-zero tutorial example.
test/rb/integration/TestClient.rb	Splits string test inputs and asserts round-trips for both escaped and unicode cases.
lib/rb/thrift.gemspec	Removes an unused local variable in gemspec setup.
lib/rb/spec/uuid_validation_spec.rb	Avoids unused locals from read_field_begin return values in UUID specs.
lib/rb/spec/header_protocol_spec.rb	Avoids unused locals from protocol read APIs in header protocol specs.
lib/rb/spec/compact_protocol_spec.rb	Avoids unused locals from read_field_begin in compact protocol spec.
lib/rb/lib/thrift/union.rb	Avoids unused field-name locals when reading unions.
lib/rb/lib/thrift/struct.rb	Avoids unused field-name locals when reading structs.
lib/rb/lib/thrift/server/thread_pool_server.rb	Removes unused rescued exception variable in worker loop.
lib/rb/lib/thrift/server/nonblocking_server.rb	Removes unused rescued exception variable in accept loop.
lib/rb/lib/thrift/protocol/base_protocol.rb	Avoids unused locals when skipping struct fields.
lib/rb/lib/thrift/exceptions.rb	Avoids unused field-name locals when reading application exceptions.
lib/rb/benchmark/benchmark.rb	Removes useless accumulator reassignment and an unused local in reporting.
lib/rb/.rubocop.yml	Enables Lint/UselessAssignment to enforce unused-assignment cleanup going forward.
compiler/cpp/tests/rb/t_rb_generator_functional_tests.cc	Adds functional expectations for void/no-arg Ruby service codegen behavior.
compiler/cpp/src/thrift/generate/t_rb_generator.cc	Updates Ruby code generation to avoid unused args/result locals in void/no-arg scenarios.

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