docs: add conformance summary to README

[matteoroxis]

Add a conformance summary section to the root README and link to the detailed validation results.

Motivation and Context

This change addresses issue #798, which requests adding the conformance test results to the README.

The goal is to make the current MCP Java SDK conformance status easier to discover directly from the repository homepage, while keeping the full details in conformance-tests/VALIDATION_RESULTS.md.

How Has This Been Tested?

This is a documentation-only change.

The values added to the README were taken from the existing conformance-tests/VALIDATION_RESULTS.md file and summarized there without changing any code or test behavior.

Breaking Changes

No breaking changes.

This update only affects documentation and does not require users to change code or configuration.

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

This PR adds a short conformance summary for:

• Server conformance
• Client conformance
• Auth conformance

and links readers to conformance-tests/VALIDATION_RESULTS.md for the full validation report.