Add a RuboCop cop forbidding non-type assertions in test/stdlib

[ksss]

Tests under test/stdlib exist to catch mismatches between RBS signatures and the runtime behavior: a test there should fail when the RBS definition is wrong. Assertions like assert_equal or assert_raises keep passing when the signature is broken, yet they keep showing up in new tests (#3216 removed the existing ones).

This adds StdlibTest/NonTypeAssertion, a local cop loaded via require: in .rubocop.yml.

• Applies to test/stdlib/**/*_test.rb only.
• Reports every receiver-less assert* / refute* call (and method(:assert...) references), except:
  • methods listed in AllowedMethods: assert_send_type, assert_send_type_error, refute_send_type, assert_const_type, assert_visibility, assert_type
  • helpers defined with def in the same file (their bodies are checked too)
• An allowlist rather than a denylist, so aliases such as assert_not_nil cannot slip through.
• test/stdlib/constant_drift_test.rb is excluded since its assert_empty compares RBS declarations with runtime constants.
• The offense message tells the author what to use instead (assert_send_type, assert_send_type_error, assert_const_type).

Existing offenses are fixed:

• assert_raises { assert_send_type ... } in the URI tests is rewritten with assert_send_type_error.
• The Proc#call arity checks in Hash_test are removed, as they do not depend on RBS.

rubocop/ is excluded from the gem files. The cop's tests live in test/rubocop/ and run with rake test.

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