test: lock connected image inline in assemble_retrieval_results

[Frankie-Xu]

Related to #206. Supersedes #331.

Summary

• The silent connect_to image drop in assemble_retrieval_results is already fixed on main by feat: table-embedded images, size filter, and doc_nav top_summary #221 (_connected_display_by_target handles image and table; missing placeholders are appended).
• test: regression coverage for connected image/table hydration #331 added a parallel test module. This PR instead adds two assemble-level regressions to the existing test_asset_inline.py:
  • image with connect_to and no placeholder in the parent text
  • image inlined at a [images/...] placeholder
• No production code change.

Test plan

• make lint
• uv run pytest packages/shared-python/shared/tests/test_asset_inline.py -q (7 passed)

Made with Cursor

[Frankie-Xu]

Rebased onto current main (d6f45c7f, includes #410).

Kept both assemble-level image regressions from this PR:

• connected image without a placeholder (Connected image chunks silently dropped in assemble_retrieval_results #206 silent-drop lock)
• image inserted at the placeholder

Resolved the conflict with #410's allowed_chunk_types filter tests. Please approve Actions workflows for this first-time fork PR so CI can run.