Adding test for streamable errors for StreamableHTTPTransport

[mogox]

Adding test for two error cases to increase test code coverage

Motivation and Context

I'm getting familiar with the gem for a project and wanted to contribute back by increasing the test coverage

How Has This Been Tested?

Adding test and running rake test. Locally I installed simple-cov and took a look at which methods didn't had coverage.

Breaking Changes

No changes on feature code

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

Additional context