Increase test coverage for more middleware scenarios

[ondrejsevcik]

The current test suite is missing coverage for several important scenarios:

Missing test scenarios:

• Handling of empty middlewares array vs. undefined middlewares
• Custom fetchFn parameter usage and behavior
• Error propagation through the middleware chain
• Middleware that does not call next()
• Middleware that transforms request or response
• Async middleware execution and edge cases
• Support for multiple arguments to fetch (e.g., url, options)

Proposed actions:

• Add test cases for each scenario listed above
• Ensure all edge cases and error paths are covered
• Refactor existing tests for clarity and completeness where necessary

Improving coverage for these cases will help ensure the middleware behaves correctly under all expected and edge-case conditions.