Adding an extension field

The extension specification, config/schemas/systemone/vllm-sr-5.0.extensions.yaml, defines every field vllm-sr adds to the standard once: its level, its schema, the version it appeared in, the families that honour it, and what a standard target does with it.

1. Add the entry. Give it its level (request, question, answer, ...), since, families, schema and summary, and its standard_target: strip when its standard part carries the meaning, refuse when a rule that needs it must fail to load, absent for a field only a vllm-sr server sends. Prefer the standard's own form: a field that overlaps a standard one is refused, and the generator fails on a field both specifications define differently.
2. Regenerate. make systemone-schemas-generate writes the Go field tables, the model runtime's field tables (vllm_srun/api/systemone_fields.py), the strip list a standard target uses, and the generated extension reference and compatibility matrix. make systemone-schemas-check fails on a stale output.
3. Implement it where it is read: the model runtime families that honour it, the Router's codecs when the Router sends or reads it, and a new minor version of the extension set (extension_version) when it is the first addition since a release.
4. Test it through both listeners and the client codecs: a 5.0 server accepts it, a standard target strips or refuses it as declared, and an unknown neighbour is still refused.