test: Switch to Minitest Test Doubles

[arielvalentin]

⚠️ This PR was generated using copilot

The tests are currently failing with System Stack errors. This PR is a placeholder for what it would look like if we chose minitest mocks over rspec.

AI Generated comments below

Minitest 6 extracted test doubles into its own gem (minitest-mock).

This is an alternative approach to #2011 (which used rspec-mocks) for comparison purposes. Instead of adding rspec-mocks, this PR uses the official minitest-mock gem — the extracted version of Minitest's built-in mock/stub functionality.

cc: #1908

Changes

• Replace rspec-mocks ~> 3.13.7 with minitest-mock ~> 5.0 in 30 Gemfiles
• Update 29 test_helper.rb files to require 'minitest/mock' instead of rspec/mocks/minitest_integration
• Convert all allow/expect RSpec Mocks patterns to Minitest stub blocks across 25 test files

Conversion Patterns

RSpec Mocks	minitest-mock
allow(X).to receive(:m).and_return(v)	X.stub(:m, v) { ... }
allow(X).to receive(:m) { block }	X.stub(:m, ->() { block }) { ... }
expect(X).not_to receive(:m)	X.stub(:m, ->(*) { flunk }) { ... }
expect(X).to receive(:m).exactly(N).times	Counter variable + assert_equal N, count
and_wrap_original	Capture .method(:name) + lambda stub
and_call_original	Implicit via stub block scope

Trade-offs vs #2011 (RSpec Mocks)

Pros:

• Stays within the Minitest ecosystem — no RSpec dependency
• minitest-mock is the official extraction maintained by the Minitest team
• Same API as Minitest 5's built-in stubs (familiar to existing contributors)

Cons:

• Block-scoped stubs increase nesting (each stub wraps remaining test code)
• No built-in spy/verification — requires manual tracking variables
• No and_wrap_original equivalent — requires capturing .method(:name) manually

For Reviewers

The main structural difference is block-scoped stubs. I recommend reviewing with Hide whitespace changes:

https://github.com/open-telemetry/opentelemetry-ruby-contrib/pull/new/fix/minitest-6-minitest-mock?w=1