fix(slack): guard against empty text in chat.postMessage (#52663)

[liuhao1024]

What does this PR do?

Guards against Slack no_text API errors when chat.postMessage is called with empty or whitespace-only text. This occurs when cron delivery posts an agent response before the agent produces content (e.g. a malformed turn before [SILENT]).

Related Issue

Fixes #52663

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• plugins/platforms/slack/adapter.py: Add early-return empty-text guard in _standalone_send() (out-of-process cron delivery) and send() (in-process gateway adapter). Both paths now skip the API call and return success when formatted text is empty/whitespace.
• tests/gateway/test_slack.py: Add TestEmptyTextGuard class with 4 tests covering both empty string and whitespace-only inputs for both send paths.

How to Test

1. Run pytest tests/gateway/test_slack.py::TestEmptyTextGuard -xvs — all 4 tests should pass.
2. Run pytest tests/gateway/test_slack.py -q — all 213 tests should pass (no regressions).
3. Run pytest tests/tools/test_send_message_tool.py -q — all 146 tests should pass (standalone sender integration).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: plugins/platforms/slack/adapter.py (_standalone_send, send)
• Blast radius: LOW — additive guard, no existing behavior changed
• Related patterns: Other adapters (Discord, Telegram) have similar empty-text handling at the base adapter level

[teknium1]

Thanks for addressing the verified Slack cron-delivery failure. The proposed guards cover the normal send() path and the standalone sender, but one direct chat_postMessage path remains.

Problems

• plugins/platforms/slack/adapter.py:3547-3559 posts result_text from slash_confirm.resolve() whenever it is truthy. tools/slash_confirm.py:140 returns any handler result that is a str, so a whitespace-only result passes if result_text and is posted to Slack, reproducing the same no_text failure class.

Suggested changes

• Apply the same whitespace check to the slash-confirm follow-up before its direct chat_postMessage call, ideally via a shared predicate, and add a regression test for a whitespace-only resolved result.

Automated hermes-sweeper review.

[teknium1]

Merged via #69317 — your commit was cherry-picked onto current main with your authorship preserved in git history: your empty-text postMessage guard was cherry-picked directly.

Thanks for the contribution!