…nds (NousResearch#17261)

The cross-channel `send_message` tool's `_send_to_platform` had no
Slack-with-media branch — Slack fell through to the "non-media platforms"
block and dropped attachments with the misleading warning:

    MEDIA attachments were omitted for slack; native send_message media
    delivery is currently only supported for telegram, discord, matrix,
    weixin, signal and yuanbao

This is the bug from NousResearch#17261: in-thread agent replies upload media fine
because `BasePlatformAdapter._process_message_background` dispatches
through the SlackAdapter's `send_image_file` / `send_voice` / `send_video`
/ `send_document` (all backed by `files_upload_v2`), but cross-channel
and DM sends through the `send_message` tool never enter that pipeline.

This change:

- Adds a `Platform.SLACK and media_files` branch in `_send_to_platform`,
  modeled on the existing Telegram/Discord branches: media attaches to
  the final chunk only.
- Extends `_send_slack(token, chat_id, message, media_files=None,
  thread_id=None)` to call `slack_sdk.web.async_client.AsyncWebClient.
  files_upload_v2` directly when media is present — the same API the
  SlackAdapter uses internally. The text becomes the first upload's
  `initial_comment`; subsequent uploads in the same call don't duplicate
  it. `thread_id` is forwarded as `thread_ts` so in-thread cross-channel
  sends still land in the right thread.
- Updates the "MEDIA attachments were omitted" warning + the
  media-only-no-text error to list slack as a supported platform.

Approach choice: directly using `AsyncWebClient(token=...)` mirrors the
one-shot HTTP/SDK style of the other senders in `send_message_tool.py`
(no long-lived adapter, no conflict with the running gateway). The
`SlackAdapter` itself remains untouched.

Tests:
- `TestSendSlackMedia` (6 tests): covers single image, thread_ts
  forwarding, multi-file caption-on-first-only invariant, missing-file
  error, `SlackApiError` surfacing, and a regression test that the
  text-only path does NOT import `slack_sdk` (gates the new dep
  behind `media_files`).
- `TestSendToPlatformSlackMedia` (4 tests): covers media branch
  routing, last-chunk-only attachment for chunked messages, text-only
  Slack still hits the existing `chat.postMessage` path, and the
  warning text now lists slack.

Regression guard: with `tools/send_message_tool.py` reverted to
origin/main (58a6171) but the new tests in place, the three
representative tests fail (`test_single_image_upload_uses_files_upload_v2`,
`test_media_present_uses_send_slack_with_media`,
`test_warning_message_lists_slack_as_supported`); restoring the fix
makes all 97 tests in the file pass and the 228 tests across
`tests/gateway/test_slack*.py` continue to pass.

Fixes NousResearch#17261

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

… path

Addresses both Copilot inline findings on NousResearch#17348:

1. **Proxy not applied to AsyncWebClient (line 1012):** `files_upload_v2`
   was bypassing the proxy that the rest of the Slack stack uses. Text-only
   `chat.postMessage` goes through `resolve_proxy_url()` +
   `proxy_kwargs_for_aiohttp()`, and SlackAdapter applies a NO_PROXY-aware
   resolver to every Slack SDK client it constructs. The new media upload
   path needs the same treatment or uploads silently fail in proxied
   environments. Now imports `_resolve_slack_proxy_url` and
   `_apply_slack_proxy` from `gateway.platforms.slack` and applies the
   resolved proxy to the throwaway `AsyncWebClient`. The import is wrapped
   so the slack_sdk-only code path still works if `gateway.platforms.slack`
   (which depends on slack_bolt) is unavailable.

2. **Text-only Slack path ignores thread_id (line 541):** `_send_to_platform`
   accepts `thread_id` but the Slack text-only branch was calling
   `_send_slack(token, chat_id, chunk)` without passing it. So a caller
   like `send_message(target='slack:C123:1700000000.1')` would land in the
   channel root, not the thread. Now forwarded as `thread_ts` (matching
   the existing media path).

Test changes:
- Added `test_text_only_slack_forwards_thread_id` regression test for
  finding 1.
- Added `test_proxy_resolved_via_slack_helpers_is_applied_to_client` and
  `test_no_proxy_resolved_does_not_set_client_proxy` for finding 2 — both
  verify the SlackAdapter proxy helper is consulted and the client's
  `.proxy` attribute is only set when a proxy is actually resolved.
- Updated `test_slack_messages_are_formatted_before_send` to expect the
  new `thread_id=None` kwarg (was positional-only assertion).
- Loosened `test_text_only_slack_still_uses_text_branch` to assert
  "no media_files kwarg" rather than "no kwargs at all" since thread_id
  is now always passed through.

All 100 tests in `tests/tools/test_send_message_tool.py` pass; all 247
tests across the Slack adapter suites and `test_send_message_missing_platforms.py`
still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>