fix(coding-agent): clear delivered image-only queue entries

[wutongyuonce]

Summary

• remove delivered queue entries even when the user message has no text
• keep image-only steering and follow-up pending counts in sync with the agent queue
• cover image-only steering delivery with a regression test

Testing

• npm run check
• ./test.sh

Fixes #8581

[gaoanze888]

Reviewed exact head b67b3db2a. Removing the non-empty-text guard is correct because the session-side queues store text only, including "" for image-only messages, while delivery is authoritatively signaled by the user message_start event.

I checked both queue modes and the cross-queue ordering. Agent core always drains steering before follow-up, matching this handler's steering-first removal; multiple image-only entries are removed one at a time in FIFO delivery order. Initial image-only prompts cannot remove anything because no session queue record exists, and aborted/undelivered entries are not cleared because no corresponding user message_start occurs.

Validation:

• queue suite plus local image-only follow-up / mixed steering+follow-up matrix: 17/17
• exact-head merge into current origin/main applies cleanly
• git diff --check

The old PR base's full build is currently affected by live generated-model/catalog drift in untouched AI provider files; the focused coding-agent lifecycle tests pass. No blocker found.

This comment is AI-generated by /wr

[wutongyuonce]

Hi @mitsuhiko @badlogic — gentle bump. This PR is still open and mergeable. Happy to adjust anything once you have a chance to look. Thanks!