feat: add channel-level echo suppression spec points

specifications/features.md

@@ -707,6 +707,9 @@ The threading and/or asynchronous model for each realtime library will vary by l
     - `(RTL4k1)` If any channel parameters are requested (which may be through the `params` field of the `ATTACH` message or some other way opaque to the client library), the `ATTACHED` (and any subsequent `ATTACHED` s) will include a `params` property (also a `Dict<String, String>`) containing the subset of those params that the server has recognised and validated. This should be exposed as a read-only `params` field of the `RealtimeChannel` (or a `getParams()` method where that is more idiomatic). An `ATTACHED` message with no `params` property must be treated as equivalent to a `params` of `{}` (that is, `RealtimeChannel.params` should be set to the empty dict)
   - `(RTL4l)` If the user has specified a `modes` array in the `ChannelOptions` ([`TB2d`](#TB2d)), it must be encoded as a bitfield per [`TR3`](#TR3) and set as the `flags` field of the `ATTACH` `ProtocolMessage`. (For the avoidance of doubt, when multiple different spec items require flags to be set in the `ATTACH`, the final `flags` field should be the bitwise OR of them all)
   - `(RTL4m)` On receipt of an `ATTACHED`, the client library should decode the `flags` into an array of `ChannelMode` s (that is, the same format as `ChannelOptions.modes`) and expose it as a read-only `modes` field of the `RealtimeChannel` (or a `getModes()` method where that is more idiomatic). This should only contain `ChannelMode` s: it should not contain flags which are not modes (see [`TB2d`](#TB2d))
+  - `(RTL4n)` If the `params` object includes an `echo` key with value `"false"`, messages originating from this connection must not be echoed back on this channel, regardless of the connection-level `echoMessages` setting ([`TO3h`](#TO3h)). The only valid value for the `echo` channel param is `"false"`. If the `echo` param is absent or empty, the connection-level `echoMessages` setting applies.
+    - `(RTL4n1)` The `echo` channel param only affects messages (action `MESSAGE`). Presence messages must always be delivered regardless of the `echo` setting, consistent with connection-level `echoMessages` behavior ([`RTN2b`](#RTN2b)).
+    - `(RTL4n2)` The `echo` channel param is a continuing property: changing it via `setOptions` ([`RTL16a`](#RTL16a)) on an already-attached channel must trigger a new `ATTACH` to update the server.
 - `(RTL5)` `RealtimeChannel#detach` function:
   - `(RTL5a)` If the channel state is `INITIALIZED` or `DETACHED` nothing is done
   - `(RTL5i)` If the channel is in a pending state `DETACHING` or `ATTACHING`, do the detach operation after the completion of the pending request
@@ -758,6 +761,8 @@ The threading and/or asynchronous model for each realtime library will vary by l
   - `(RTL7d)` Messages delivered are automatically decoded based on the `encoding` attribute; see `RestChannel` encoding features in [RSL6](#RSL6). Tests should exist to publish and subscribe to encoded messages using the fixture test data referenced in [RSL5c](#RSL5c).
   - `(RTL7e)` This clause has been deleted (redundant to [RSL6b](#RSL6b)).
   - `(RTL7f)` A test should exist ensuring published messages are not echoed back to the subscriber when `echoMessages` is set to false in the `RealtimeClient` library constructor
+  - `(RTL7i)` A test should exist ensuring published messages are not echoed back to the subscriber when `echo` is set to `"false"` in the channel `params` ([`RTL4n`](#RTL4n)), even when `echoMessages` is `true` (default) at the connection level.
+  - `(RTL7j)` A test should exist ensuring that on a connection with `echoMessages: false`, a channel without the `echo` param still suppresses echo (inherits connection-level behavior).
 - `(RTL8)` `RealtimeChannel#unsubscribe` function:
   - `(RTL8c)` Unsubscribe with no arguments unsubscribes all listeners
   - `(RTL8a)` Unsubscribe with a single listener argument unsubscribes the provided listener to all messages if subscribed