fix: forward Slack image attachments to queued follow-up messages

[Johan Bonilla (johanity)]

Description

Slack image attachments were silently dropped when a user sent a follow-up with an image while open-swe was already running on that thread.

process_slack_mention built the correct image_urls list from the thread context, but on the busy path hardcoded image_urls=[] in the queued payload:

# agent/webapp.py (before)
queued_payload = {"text": prompt, "image_urls": []}

check_message_queue_before_model then read an empty list and never fetched the images, so the agent saw the follow-up text but not the screenshot. Regression introduced in #1050.

The rest of the queue pipeline — including Slack-auth in fetch_image_block — was already wired correctly. Only the queue producer was missing the forward.

This PR also reorders process_slack_mention so the is_thread_active check runs before the image fetch loop. Previously, images were fetched from files.slack.com on the busy path even though the results were then discarded by the image_urls=[] queue payload.

On the busy path the queue now carries images filtered to the current mention only, not the full thread context. Re-injecting historical images would duplicate what the agent already saw in its prior turn.

Test Plan

New tests/test_slack_image_queue.py:

• Busy path forwards current mention's image URL to the queue payload
• Busy path excludes historical context images
• Busy path produces empty image_urls for a text-only follow-up
• Non-busy path still fetches images into content_blocks

Full suite: 111 passed.