Reorder StdioServerTransport shutdown procedure to avoid errors

[chemicL]

In the tests occasionally the STDIO variant would fail due to dropped errors.

Motivation and Context

When exercising the StdioMcpSyncServerTests occasionally onErrorDropped signals would be logged signifying that the shutdown of the inbound is not properly ordered with regards to the outbound and the outbound-related Scheduler is shutdown before the pipeline has a chance to process the completion signal.

How Has This Been Tested?

./mvnw clean install

Breaking Changes

No.

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

None