refactor(transport): explicitly specify UTF-8 charset when creating InputStreamReader

[codeboyzhou]

1. Make scheduler fields final in client and server transports
2. Explicitly specify UTF-8 charset when creating InputStreamReader instances
3. Remove method protocolVersions() in class StdioServerTransportProvider

Motivation and Context

1. For better immutability
2. (SpotBugs check) Ensure consistent character encoding to avoid potential problem
3. Method protocolVersions() implementation is identical to its super class, no need to override it

How Has This Been Tested?

Just run ./mvnw test

Breaking Changes

Not involved

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

Not involved