Emit required array in tool schema based on @Nullable

[christophercolumbusdog]

Summary

• Tool annotation processor now checks @Nullable on parameters (both element and TYPE_USE annotations) and emits a "required" array in the generated JSON schema for non-nullable parameters
• MCP clients can now distinguish mandatory from optional tool inputs

Fixes #146

Changes

• McpCodegenUtil — add isNullable helper that checks both annotations() and elementTypeAnnotations() for any annotation named Nullable
• McpJsonSchemaCodegen — add overload of addSchemaMethodBody that accepts required field names and emits "required": [...] between the properties close and root object close
• McpToolCodegen — collect non-nullable parameter names during schema generation and pass them through

Example

Given:

@Mcp.Tool("Abandon a change")
public List<McpToolContent> abandonChange(
        @Mcp.Description("Change ID") String changeId,
        @Mcp.Description("Reason") @Nullable String message) { ... }

Before:

{"type": "object", "properties": {"changeId": {...}, "message": {...}}}

After:

{"type": "object", "properties": {"changeId": {...}, "message": {...}}, "required": ["changeId"]}

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