Add structured provider output

[chubes4]

Summary

• add a normalized JSON Schema structured-output contract at the provider-turn boundary
• preserve parsed JSON object, list, scalar, and null shapes through the conversation and canonical chat results
• map bare wp-ai-client turns through its public JSON output APIs; strict mode validates the documented local schema subset and returns redacted typed failures
• fail closed for tool or streaming combinations and redact public diagnostics

Verification

• composer validate --no-check-publish
• composer phpstan
• composer smoke
• focused provider-turn, default adapter, chat schema, and default chat handler smoke suites

Fixes #516

AI assistance

OpenCode with openai/gpt-5.6-sol implemented and revised the provider-neutral contract, inspected the installed wp-ai-client APIs, and added focused tests. Chris Huber reviewed and remains responsible for the change.