fix(slack-gateway): preserve ACP reply whitespace

[onutc]

Summary

• preserve ACP chunk whitespace and newlines in the Slack gateway reply path
• move ACP text extraction/joining into a shared lossless helper used by the API and Slack gateway
• add regression coverage for chunk-boundary whitespace, final Slack payload integrity, and document the invariant

Validation

• go test ./... (workdir: acptext)
• go test ./... (workdir: integrations/slack-gateway)
• go test ./... (workdir: api)
• git diff --check
• npx -y @simpledoc/simpledoc check

[onutc]

Superseded by clean PR #195. Closing this one because #194 includes unrelated changes from its original branch ancestry.