REST hardening: guest-author creation and post-insert observability

[dknauss]

Summary

• harden guest-author username handling in the REST user endpoint
• improve collision handling when generating guest-author usernames
• add failure observability for post insert author assignment path
• extend tests for both guest-author creation and post-save error handling

Files

• inc/class-users-controller.php
• inc/class-insert-post-handler.php
• tests/phpunit/test-rest-api-user-endpoint.php
• tests/phpunit/test-post-saving.php

Verification

• composer test:ut (pass; upstream test suite currently emits deprecation/risky noise under PHP 8.5)

Notes

• This PR intentionally excludes CLI and editor changes.

[dknauss]

Fork status update (2026-03-08): fork-first delivery continues on dknauss/authorship:develop.

• Current fork execution state: https://github.com/dknauss/authorship/blob/develop/docs/audit/roadmap-global.md
• Fork development changelog: https://github.com/dknauss/authorship/blob/develop/CHANGELOG.md

Keeping this PR open as an optional upstream adoption path; fork delivery is not gated on upstream merge timing.

[dknauss]

Material fork update (2026-03-08):

• Phase 03 remains complete fork-locally.
• NVDA transcript capture is tracked as residual follow-up in fork issue User capability mapping #11: Residual follow-up: capture NVDA transcript for UI-06 AT matrix dknauss/authorship#11
• Traceability + Phase 04 kickoff-prep docs are in fork PR Remove the default "Author" control on the block editor screen #12: Docs: trace NVDA follow-up and prep Phase 04 kickoff dknauss/authorship#12

This upstream PR remains open as an optional adoption path; fork delivery continues independently.

[dknauss]

Final housekeeping update (2026-03-08):

• Fork strict policy is now merged and canonical at:
  https://github.com/dknauss/authorship/blob/develop/docs/fork-first-policy.md
• Fork roadmap/policy docs are aligned to this model:
  • fork develop is delivery source of truth
  • upstream PRs are optional adoption paths
  • upstream PR sprawl has been reduced; only packaging set remains open

This PR remains open as an optional upstream adoption path.