fix wrong role default value

[JavaProgrammerLB]

Motivation and Context

• fix typo in SamplingRequest.tsx which config default role(should choose from 'user' or 'assistant') value to 'endTurn'

How Has This Been Tested?

when I update role to 'endTurn' and click 'approve' I met the error('endTurn' is not allowed value)

when I update role to 'assistant' and click 'approve', everthing goes well

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