fix: handle synchronous exceptions in QueuedInterceptor

[Danilospano00]

Fixes a bug where QueuedInterceptor would cause requests to hang indefinitely when an interceptor callback threw a synchronous exception.

The _handleQueue method now accepts an onError callback that properly completes the handler when a synchronous exception is caught, ensuring requests fail gracefully instead of hanging.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

Description

Fixes a bug where QueuedInterceptor would cause requests to hang indefinitely when an interceptor callback threw a synchronous exception.

Problem

When using QueuedInterceptor, if an interceptor callback (onRequest, onResponse, or onError) threw a synchronous exception (not wrapped in a Future), the request would hang until timeout. This occurred because the _handleQueue method's try-catch block caught the exception but only called _processNextInQueue() without completing the handler's Completer, leaving the request waiting forever.

Changes

Modified _handleQueue to accept an onError callback parameter that properly handles synchronous exceptions:

• onRequest/onResponse: Wraps the error in DioException and calls handler.reject() to fail the request gracefully
• onError: Wraps the error in DioException and calls handler.next() to pass the error to the next interceptor in the chain

[AlexV525]

Thanks for the fix. I was wondering:

1. Can the code add more comments to tell people what happens at this place?
2. Does it also fix A request with an interceptor, which produces an error, makes the code stuck. #2138, which fix: Catch sync/async* exceptions in interceptor's handlers #2139 didn't make it?

[Danilospano00]

Thanks for the review!

I've added some comments.

This PR fixes the same type of issue as #2138, but for QueuedInterceptor instead of only the regular Interceptor.

I've also added test that reproduces the #2138 scenario with QueuedInterceptor to verify that the fix works correctly.

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
dio/lib/src/interceptor.dart	🟢 97.66%	🟢 98.6%	🟢 0.94%
Overall Coverage	🟢 87.95%	🟢 88.1%	🟢 0.15%

Minimum allowed coverage is 0%, this run produced 88.1%