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>

docs/.generated/config-baseline.sha256

@@ -1,4 +1,4 @@
-27444d825e0dda91c769267beb81b3826ca42fd9c2e16d4d6bedccbc5cbccba4  config-baseline.json
+bb58618ecdd5cb16d985eafa4a3a4634f366e6674f3db16c246336e6e02a3706  config-baseline.json
 b95cc4a3b15e688afa58cf700dac425af4e50fd092209e75903e73d5d2988717  config-baseline.core.json
-07005d122bda68081d530ae5cd455ffbc0817f7fb94bd462c4f25edc94a0546c  config-baseline.channel.json
+f2a1aad257c570b497865680c331568a6775369528749826dfa35c1f644483fc  config-baseline.channel.json
 fffe0e74eab92a88c3c57952a70bc932438ce3a7f5f9982688437f2cdaee0bcb  config-baseline.plugin.json