Add docs with more details on testing tools and update behavior for optional values

[olaservo]

This is related to a few recurring issues and recent changes with how the Tools UI is working (or not) in certain scenarios. The intention of this doc is to call out more explicitly what is by design, and what to expect in the UI when using Inspector when testing different types of input parameters.

How Has This Been Tested?

Tests are updated, but doc needs to be reviewed to ensure its consistent with what we actually want to happen in the UI.

Breaking Changes

See: docs/tools-testing.md for proposal for expected behavior.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation update

Checklist

• [x] I have read the MCP Documentation
• [x] My code follows the repository's style guidelines
• [x] New and existing tests pass locally
• [ ] I have added appropriate error handling
• [x] I have added or updated documentation as needed