[Server] Allow configuring the schema validator

[steffans]

Adds Builder::setSchemaValidator() to configure the validator used for tools/call input-schema validation. The
configured validator is passed to CallToolHandler; without configuration, the existing default validator behavior
remains unchanged.

Motivation and Context

Custom schema resolver configuration, such as:

$resolver->registerPrefix('http://example.com/', '/path/to/dir');

allows schemas to resolve from local directories.

How Has This Been Tested?

Added a unit test that configures a custom SchemaValidator, performs a tool call, and verifies that the supplied
validator is invoked.

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

[chr-hertel]

Thanks @steffans!