fix(slack): enable preview streaming in flat DMs (replyToMode: off) (#76330)

Summary:
- The PR enables Slack draft preview streaming for flat DMs in all non-off modes, updates Slack streaming tests/docs/config metadata/changelog, and refreshes small guard baselines.
- Reproducibility: yes. source-reproducible on current main: the helper returns false for `mode: "partial"` in ...  current test asserts that disabled path. I did not run tests because this review was explicitly read-only.

Automerge notes:
- Ran the ClawSweeper repair loop before final review.
- Included post-review commit in the final squash: fix(slack): enable preview streaming in flat DMs (replyToMode: off)
- Included post-review commit in the final squash: fix(clawsweeper): address review for automerge-openclaw-openclaw-5654…

Validation:
- ClawSweeper review passed for head 52e5d74ef9.
- Required merge gates passed before the squash merge.

Prepared head SHA: 52e5d74ef9
Review: https://github.com/openclaw/openclaw/pull/76330#issuecomment-4365017023

Co-authored-by: clawsweeper <274271284+clawsweeper[bot]@users.noreply.github.com>
Co-authored-by: bob <637186+HangGlidersRule@users.noreply.github.com>

scripts/check-no-raw-channel-fetch.mjs

@@ -18,8 +18,8 @@ const allowedRawFetchCallsites = new Set([
   bundledPluginCallsite("bluebubbles", "src/types.ts", 204),
   bundledPluginCallsite("browser", "src/browser/cdp.helpers.ts", 268),
   bundledPluginCallsite("browser", "src/browser/client-fetch.ts", 192),
-  bundledPluginCallsite("chutes", "models.ts", 535),
-  bundledPluginCallsite("chutes", "models.ts", 542),
+  bundledPluginCallsite("chutes", "models.ts", 536),
+  bundledPluginCallsite("chutes", "models.ts", 543),
   bundledPluginCallsite("discord", "src/monitor/gateway-plugin.ts", 417),
   bundledPluginCallsite("discord", "src/monitor/gateway-plugin.ts", 483),
   bundledPluginCallsite("discord", "src/voice-message.ts", 298),
@@ -30,7 +30,7 @@ const allowedRawFetchCallsites = new Set([
   bundledPluginCallsite("github-copilot", "login.ts", 69),
   bundledPluginCallsite("github-copilot", "login.ts", 101),
   bundledPluginCallsite("googlechat", "src/auth.ts", 83),
-  bundledPluginCallsite("huggingface", "models.ts", 142),
+  bundledPluginCallsite("huggingface", "models.ts", 143),
   bundledPluginCallsite("kilocode", "provider-models.ts", 130),
   bundledPluginCallsite("matrix", "src/matrix/sdk/transport.ts", 112),
   bundledPluginCallsite("microsoft-foundry", "onboard.ts", 479),