Ignore unexpected properties when deserializing a ContentBlock

[jeffhandley]

Fixes #955 - Skip unexpected properties when deserializing ContentBlock

Motivation and Context

This issue was encountered and reported by a colleague working with a server that returned data in an old/unexpected structure.

How Has This Been Tested?

Repro console app and a failing unit test were created. The new tests now succeed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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

[jeffhandley]

@stephentoub / @eiriktsarpalis - After fixing the reported case, Copilot and I looked at all other JsonConverter<T> implementations and found several others where we're not skipping unknown props. It seemed to me we should be defensive in all of these cases, but I wasn't sure if it would be more appropriate to throw an InvalidDataException in any of these cases.

While looking through this, Copilot found that Reference.Title was getting dropped on the floor too.

All of these extra fixes are in a separate commit from the original fix, in case you think we should drop the auxiliary changes.