Follow-ups on top of #98964's cherry-pick:
- PHOTON_READ_RECEIPTS env toggle (default true) so users can keep
messages at Delivered; declared in plugin.yaml optional_env
- adapter drops both 'read' and 'read_receipt' content types (alias
coverage from #91759 by @mooserini) + regression test
- docs: photon.md feature note + environment-variables.md row