fix: disambiguate input-required results with metadata

[DaleSeo]

Fixes #1094.

Motivation and Context

ServerResult is an untagged Serde union, and CallToolResult comes before InputRequiredResult. Since the CallToolResult deserializer accepted _meta without checking resultType, an intermediate result with metadata was incorrectly treated as an empty, completed tool call. As a result, its input requests and request state were dropped.

This PR prevents metadata-bearing InputRequiredResult values from being silently deserialized as completed tool results. It preserves legacy tool results that don't include resultType and adds regression coverage for both wire formats.

How Has This Been Tested?

Added coverage

Breaking Changes

None.

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