Summary

This PR fixes an issue where onerror was being triggered twice when
client.close() was executed.
The duplicate call originated from _startOrAuthSse, where an extra
this.onerror?.(error as Error); invocation existed in the catch block.

Since the method already throws the error, the extra onerror call was redundant
and caused duplicate event firing.

Motivation and Context

Fixes #868
When client.close() was executed, onerror was being triggered twice.
This happened because _startOrAuthSse had an extra this.onerror?.(error as Error); inside its catch block, even though the error was already propagated via throw error;.
By removing this line, the onerror event now fires only once, matching expected behavior.

How Has This Been Tested?

Ran the provided reproduction script from Issue #868.
Verified that after the change, the output contains only one error: The operation was aborted. log.
Confirmed no side effects or missing error propagation in other scenarios.

Breaking Changes

No breaking changes — this only removes redundant error event firing.

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