fix(slack): support MEDIA attachments in send_message

[HexLab98]

Summary

• Slack agents could already send files in-channel via gateway MEDIA: delivery (files_upload_v2), but send_message explicitly omitted Slack media and reported it as unsupported — so models inconsistently refused PDF/file sends (including after succeeding earlier in the same thread).
• Wire Slack into the native send_message media path: plugin _standalone_send uploads via files_upload_v2, with caption-on-upload for single captionable files (same pattern as Discord/WhatsApp).
• Update restriction/warning copy and tests (test_slack_send_message_media.py; Signal media tests now use SMS as the non-media platform).

Test plan

• scripts/run_tests.sh tests/tools/test_slack_send_message_media.py tests/tools/test_signal_media.py -q
• In a live Slack gateway session, ask the agent to send_message a local PDF with MEDIA:/path/file.pdf to a channel and confirm the file appears
• Confirm caption-only single-file sends put text in initial_comment (no duplicate text message)
• Confirm in-channel reply MEDIA: delivery still works unchanged

[alt-glitch]

This was generated by AI during triage.

Related to #17261, #21800, and #23913. This uses the plugin standalone sender and files_upload_v2; the other open implementations take different sender/upload paths.

[teknium1]

Thanks for wiring the missing Slack media route. The premise is confirmed on current main: tools/send_message_tool.py:1035-1063 classifies Slack as non-media and invokes its standalone sender without media_files.

Problems

• plugins/platforms/slack/adapter.py:5007 constructs the media AsyncWebClient without the existing _resolve_slack_proxy_url() / _apply_slack_proxy() setup. Current Slack clients apply that configuration at plugins/platforms/slack/adapter.py:1080-1082; media delivery would bypass configured proxy and NO_PROXY behavior.

Suggested changes

• Apply _apply_slack_proxy(client, _resolve_slack_proxy_url()) after constructing the media client, and add coverage for that path.

Automated hermes-sweeper review.

[teknium1]

This media client bypasses Slack's established proxy setup. Please apply _apply_slack_proxy(client, _resolve_slack_proxy_url()) here, as the connected clients do at current-main adapter.py:1080-1082, and add a proxy-path test.

[teknium1]

Merged via #69482 — your commit was cherry-picked/reapplied onto current main with your authorship preserved in git history: your MEDIA-in-send_message implementation (caption-on-upload, per-file warnings, chunked routing, tests) won the three-way adjudication and was cherry-picked directly.

Thanks for the contribution!