fix(slack): preserve legacy attachment URLs

[jcarugati]

What does this PR do?

Preserves HTTP(S) URLs carried by legacy Slack attachments when rendering fetched thread context and thread-parent reply context.

Current main already merges visible legacy attachment text and top-level Block Kit URLs. However, its URL walker only receives top-level blocks, so attachment fields such as title_link, author_link, and from_url—plus URLs in attachment-nested Block Kit—are omitted even though the attachment title/text remains visible.

This follows up on #69316 and keeps the same renderer, URL filtering, and discovery-order deduplication.

Related Issue

Follow-up to #69316. Searches for the affected legacy attachment URL fields found no open issue or equivalent PR.

Type of Change

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

Changes Made

• Generalize the existing Slack URL walker to cover both top-level Block Kit and legacy attachment URL fields.
• Feed the renderer's blocks and attachments through that single walker.
• Add a regression test for title_link, author_link, from_url, and attachment-nested Block Kit URLs.

How to Test

1. Render a Slack message whose visible title/text is in a legacy attachment and whose links are only in attachment URL fields or nested Block Kit.
2. Confirm all attachment URLs appear in _render_message_text output alongside existing top-level Block Kit URLs.
3. Run the focused Slack suite below.

Validation

• scripts/run_tests.sh tests/gateway/test_slack*.py -q — 544 passed
• .venv/bin/ruff check . — passed
• .venv/bin/python scripts/check-windows-footguns.py --all — passed
• .venv/bin/python -m compileall -q plugins/platforms/slack/adapter.py tests/gateway/test_slack_approval_buttons.py — passed
• git diff upstream/main...HEAD --check — passed

Compatibility

• No configuration, dependency, session, or outbound Slack payload changes.
• URL collection remains limited to HTTP(S) and preserves first-seen order with duplicate removal.
• The change is platform-neutral dictionary traversal.

Checklist

• I've read the contribution guide and repository instructions.
• My commit uses the repository's Conventional Commit format.
• I searched open and closed issues/PRs for an equivalent fix.
• The PR contains only this Slack thread-rendering fix and its regression test.
• Documentation, configuration, architecture, and tool-schema updates are N/A.
• Cross-platform impact was considered and the repository's Windows footgun check passes.

[teknium1]

Thanks for the focused Slack rendering fix. The premise remains valid on current main: plugins/platforms/slack/adapter.py:641 recognizes only Block Kit URL keys, and _render_message_text passes only top-level blocks to that walker at plugins/platforms/slack/adapter.py:7158. The proposed generic traversal preserves the existing HTTP(S), first-seen-order, and deduplication behavior while adding the attachment fields and nested blocks that current rendering omits.

No additional correctness or design-fit changes identified. This is an automated hermes-sweeper review.