fix: Avoid NPE in McpClientSession

[vorburger]

Motivation and Context

Fixes NPE seen in the wild as described in #605.

How Has This Been Tested?

As described in #605, unfortunately I'm unable to reproduce that NPE locally even myself, but I am consistently seeing this on a failing CI build. However I'm confident that this change improves the robustness of McpClientSession and will provide a much more clear and actionable error message when this occurs again next time.

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? No, see Tests run: 853, Failures: 5, Errors: 9, Skipped: 8 #606 - but that's unrelated to this change.
• I have added appropriate error handling
• I have added or updated documentation as needed