Use pydantic MISSING sentinel to mark optional but non-nullable fields

[edgarrmondragon]

Links

• https://pydantic.dev/docs/validation/latest/concepts/experimental/#missing-sentinel
• https://docs.python.org/3.15/whatsnew/3.15.html#whatsnew315-sentinel
• https://peps.python.org/pep-0661/
• Stabilize MISSING sentinel feature pydantic/pydantic#12090
• Add support for Python 3.15 pydantic/pydantic#13173
• Add support for PEP 661 (sentinels) python/mypy#21454
• Add support for PEP 661 (sentinels) python/mypy#21647

[github-actions]

Build Errors

Plugin	Error	Value	Location

[github-actions]

	Tests	Passed ✅	Skipped	Failed
JUnit Test Report	11 ran	11 passed	0 skipped	0 failed

Test	Result
JUnit Test Report
GET /meltano/api/v1/maintainers	✅ passed
GET /meltano/api/v1/maintainers/top	✅ passed
GET /meltano/api/v1/plugins/index	✅ passed
GET /meltano/api/v1/plugins/made-with-sdk	✅ passed
GET /meltano/api/v1/plugins/search	✅ passed
GET /meltano/api/v1/plugins/stats	✅ passed
GET /meltano/api/v1/plugins/{plugin_type}/index	✅ passed
GET /meltano/api/v1/plugins/{plugin_type}/{plugin_name}--{plugin_variant}	✅ passed
GET /meltano/api/v1/plugins/{plugin_type}/{plugin_name}/default	✅ passed
GET /meltano/api/v1/maintainers/{maintainer}	✅ passed
Stateful tests	✅ passed