Migrate null type unions to anyOf in strict schemas (#5991)

[shiv07-blip]

Summary

Replaces type: ["X", "null"] unions with explicit anyOf branches in strict schemas.

Ajv allows two-element null unions in strict mode, so these can pass without being caught. This change makes the nullable types explicit:

type: ["X", "null"] → anyOf: [{ "type": "X" }, { "type": "null" }]

Type-specific keywords such as items, properties, minLength, format, and additionalProperties are moved into the non-null branch. Annotations like description, default, and title remain at the definition level.

This batch updates schemas with 1–5 null unions each, including the abc-supply-plan schemas, docker-bake, git-cliff, github-cli-config, hugo, jekyll, pubspec, rumdl, and others.

Files already covered by other open PRs are left untouched.

Validation

• npm run prettier
• npm run typecheck
• npm run eslint
• git diff --check
• node ./cli.js check
• node ./cli.js coverage

Implements #5991.

[madskristensen]

This has a merge conflict.

[shiv07-blip]

@madskristensen Okay, i am working on it.

[shiv07-blip]

@madskristensen , please review now and proceed

[hyperupcall]

Thanks. Had a question about these changes.