feat: add default value support to string, number, and integer schemas

[DaleSeo]

Closes #521

Motivation and Context

The MCP SEP-1034 specification extends elicitation by adding default value support to all primitive schema types. BooleanSchema and EnumSchema already had default fields, but StringSchema, NumberSchema, and IntegerSchema did not. This caused the conformance test elicitation-sep1034-defaults to fail because these three types could not deserialize a default field from JSON.

How Has This Been Tested?

Updated the JSON schema snapshot to reflect the new fields

Breaking Changes

None. All new fields are Option<T> with #[serde(skip_serializing_if = "Option::is_none")], so existing code that constructs these schemas without a default value continues to work unchanged.

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