Send required initialize params in the mutex probe test helper

[koic]

Motivation and Context

bundle exec rake fails on main with four errors in streamable_http_transport_test.rb:

NoMethodError: undefined method '[]=' for nil
  streamable_http_transport_test.rb:5119 (install_mutex_probe_stream)

This is a semantic conflict between two merged PRs. #451 made the server reject initialize requests that lack the required protocolVersion, capabilities, and clientInfo params with -32602. #449, merged after but developed before it, added the "writes outside the mutex" tests, whose initialize_test_session helper sends an initialize request with no params. That request is now rejected, no session is created, so @sessions[session_id] is nil and installing the probe stream raises NoMethodError.

The fix adds params: initialize_params to the helper's request body, matching the pattern used by every other initialize request in the file via InitializeParamsTestHelper.

How Has This Been Tested?

bundle exec rake now passes: 1318 runs, 0 failures, 0 errors, and RuboCop reports no offenses. The four previously failing tests are the regression coverage.

Breaking Changes

None. Test-only change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[koic]

I'll merge this to restore the main branch CI.