feat: add custom exception unwrapping in interceptors

[meylis1998]

Summary

• Adds mechanism to throw custom exceptions directly from interceptors instead of wrapping them in DioException
• Implements solution for issue Developer should throw custom exception in interceptor #1950

Changes

• Add throwInnerErrorOnFinish flag to DioException (default false)
• Add DioException.customError() factory constructor for convenient creation
• Add rejectCustomError() method to RequestInterceptorHandler, ResponseInterceptorHandler, and ErrorInterceptorHandler
• Unwrap marked exceptions in fetch() final catch block

Example Usage

// In interceptor
dio.interceptors.add(InterceptorsWrapper(
  onResponse: (response, handler) {
    if (response.statusCode == 401) {
      handler.rejectCustomError(
        UnauthorizedException('Token expired'),
        response.requestOptions,
      );
      return;
    }
    handler.next(response);
  },
));

// Caller can now catch custom exception directly:
try {
  await dio.get('/api');
} on UnauthorizedException catch (e) {
  // Now works!
}

Test plan

• Added tests for rejectCustomError() from all three handler types
• Added test verifying regular reject() still wraps in DioException
• Added tests for DioException.customError() factory
• Added tests for copyWith() preserving the flag
• Added test for callFollowingErrorInterceptor parameter
• Added test for QueuedInterceptor support
• All existing tests pass

Closes #1950