fix(streaming): add Slack streaming=false default to match Discord

PR #37303 added per-platform streaming defaults and the commit message
explicitly called out "Discord/Slack/etc. only have edit-based streaming
(repeated editMessage), which flickers and is noticeably jankier" — but
only discord.streaming=false was shipped. Slack uses the same edit-based
streaming mechanism and has the same flicker problem, yet it was left to
follow the global switch (default true when streaming is enabled).

Add "slack": {"streaming": False} to DEFAULT_CONFIG["display"]["platforms"]
alongside the Discord default. The same deep-merge semantics apply: a user
who explicitly sets display.platforms.slack.streaming: true keeps their
value unchanged. The dashboard schema gains a slack.streaming toggle
automatically since it is generated from DEFAULT_CONFIG.

Update test_per_platform_streaming_defaults.py to cover slack in all
existing assertions and rename the resolver test to reflect both platforms.

hermes_cli/config.py

@@ -1976,9 +1976,9 @@ DEFAULT_CONFIG = {
         # per platform:
         #   - Telegram has native animated draft streaming (sendMessageDraft),
         #     which is smooth, so streaming is on by default there.
-        #   - Discord/Slack/etc. only have edit-based streaming (repeated
+        #   - Discord and Slack only have edit-based streaming (repeated
         #     editMessage), which flickers and is noticeably jankier, so
-        #     streaming is off by default there.
+        #     streaming is off by default for both.
         # These are gap-fillers: a user who explicitly sets, e.g.,
         # display.platforms.discord.streaming: true keeps their value
         # (config deep-merge has user values win over defaults). The global
@@ -1987,6 +1987,7 @@ DEFAULT_CONFIG = {
         "platforms": {
             "telegram": {"streaming": True},
             "discord": {"streaming": False},
+            "slack": {"streaming": False},
         },
         # Gateway runtime-metadata footer appended to the FINAL message of a turn
         # (disabled by default to keep replies minimal). When enabled, renders

tests/gateway/test_per_platform_streaming_defaults.py

@@ -1,11 +1,11 @@
 """Per-platform streaming defaults + dashboard exposure.
 
 Streaming is smooth on Telegram (native sendMessageDraft) but flickers on
-edit-only platforms like Discord. The shipped defaults encode that:
-display.platforms.telegram.streaming=true, .discord.streaming=false. These are
-gap-fillers (user values win via deep-merge) and, because the dashboard schema
-is generated from DEFAULT_CONFIG, they automatically appear as editable toggles
-in the web UI.
+edit-only platforms like Discord and Slack (repeated editMessage). The shipped
+defaults encode that: display.platforms.telegram.streaming=true,
+.discord.streaming=false, .slack.streaming=false. These are gap-fillers (user
+values win via deep-merge) and, because the dashboard schema is generated from
+DEFAULT_CONFIG, they automatically appear as editable toggles in the web UI.
 """
 
 from __future__ import annotations
@@ -16,11 +16,12 @@ def test_default_per_platform_streaming_flags():
     plats = DEFAULT_CONFIG["display"]["platforms"]
     assert plats["telegram"]["streaming"] is True
     assert plats["discord"]["streaming"] is False
+    assert plats["slack"]["streaming"] is False
 
 
-def test_resolver_telegram_on_discord_off_when_global_enabled():
+def test_resolver_telegram_on_discord_and_slack_off_when_global_enabled():
     """With global streaming on, the per-platform defaults make Telegram stream
-    and Discord not — matching the platforms' actual streaming quality."""
+    and Discord/Slack not — matching the platforms' actual streaming quality."""
     from hermes_cli.config import DEFAULT_CONFIG
     from gateway.display_config import resolve_display_setting
 
@@ -34,18 +35,23 @@ def test_resolver_telegram_on_discord_off_when_global_enabled():
 
     assert streams("telegram") is True
     assert streams("discord") is False
-    # A platform with no default entry follows the global switch.
-    assert streams("slack") is True
+    assert streams("slack") is False
+    # A platform with no default entry still follows the global switch.
+    assert streams("matrix") is True
 
 
 def test_user_override_wins_over_default():
-    """A user who explicitly enables Discord streaming keeps their value — the
-    default false must not clobber it (config deep-merge: user wins)."""
+    """A user who explicitly enables Discord or Slack streaming keeps their value
+    — the default false must not clobber it (config deep-merge: user wins)."""
     from hermes_cli.config import DEFAULT_CONFIG, _deep_merge
 
-    user = {"display": {"platforms": {"discord": {"streaming": True}}}}
+    user = {"display": {"platforms": {
+        "discord": {"streaming": True},
+        "slack": {"streaming": True},
+    }}}
     merged = _deep_merge(dict(DEFAULT_CONFIG), user)
     assert merged["display"]["platforms"]["discord"]["streaming"] is True
+    assert merged["display"]["platforms"]["slack"]["streaming"] is True
     # Partial override must not wipe the sibling telegram default.
     assert merged["display"]["platforms"]["telegram"]["streaming"] is True
 
@@ -59,7 +65,9 @@ def test_dashboard_schema_exposes_per_platform_streaming():
 
     assert "display.platforms.telegram.streaming" in CONFIG_SCHEMA
     assert "display.platforms.discord.streaming" in CONFIG_SCHEMA
+    assert "display.platforms.slack.streaming" in CONFIG_SCHEMA
     assert CONFIG_SCHEMA["display.platforms.discord.streaming"]["type"] == "boolean"
+    assert CONFIG_SCHEMA["display.platforms.slack.streaming"]["type"] == "boolean"
     # Global streaming controls are exposed too.
     assert "streaming.enabled" in CONFIG_SCHEMA
     assert "streaming.transport" in CONFIG_SCHEMA