Add generated REST mutation fixture contract

[chubes4]

Summary

• Add public wp-codebox/rest-mutation-generated-fixtures/v1 for generic mutating REST fixture generation.
• Derive bounded disposable sandbox mutation operations from route schemas, optional collection samples, and typed generators.
• Return fixture-plan opt-ins plus explicit confidence, source, boundedness, semantic-validity, artifact, and unsupported-reason metadata.
• Advertise the schema through the WordPress fuzz runtime descriptor and public API docs.

Tests

• npx tsx tests/fuzz-fixture-plan-contracts.test.ts
• npm run test:public-api-contract
• npm run build

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Implementing the generated REST mutation fixture contract, tests, docs, and PR preparation.