Schema updates

[LittleCoinCoin]

This pull request introduces Hatch Package Schema version 1.2.1, which adds dual entry point support for packages and strengthens tool validation requirements. The documentation has been updated to reflect these changes, including migration guidance, new examples, and an updated JSON schema. These improvements enhance separation of concerns, external compatibility, and package validation across the ecosystem.

Schema and Validation Enhancements

• Added new schema file package/v1.2.1/hatch_pkg_metadata_schema.json with support for dual entry points (mcp_server and hatch_mcp_server) and enforced tool declaration requirements.
• Updated documentation and examples to require and demonstrate the dual entry point format and stricter tool validation. [1] [2] [3]

Documentation Updates

• Updated all references to the latest schema version (v1.2.1) in README.md and docs/package/overview.md, including versioned schema lists and example metadata. [1] [2]
• Clarified the schema's entry point section to describe dual entry point configuration and its benefits.

Migration Support

• Added a comprehensive migration guide (docs/package/v1.2.1_migration_guide.md) detailing the steps and requirements for upgrading packages from v1.2.0 to v1.2.1, including code samples and troubleshooting tips.