feat(plugin): add schema-v3 stream chunk contract to omit payload request bodies

- Bump plugin schema to version 3 and introduce `SchemaVersionStreamChunkOmitRequestBody`.
- Treat missing plugin schema versions as legacy during RPC registration (`0 -> 1`) and expose schema on plugin descriptors.
- In stream interception, keep request headers/bodies on header-init chunk and stop re-sending them on payload chunks for schema-v3+ plugins, with per-chunk cloning for legacy plugins.

Closes: #4876