Preserve ordered mixed content in agent messages without disrupting text
streaming for existing native/v1 agents. The previous single-block
reduction could replace images with placeholders or lose non-text
content as subsequent chunks arrived.

- Introduce [ordered message display
content](https://github.com/zed-industries/zed/blob/929fba3e16f7a378eb1dca3d85ac2f1df2b902c6/crates/acp_thread/src/acp_thread.rs#L1500),
reusing adjacent text entities while retaining image/resource-preview
boundaries.
- Update rendering, text search, response copy/export, Mermaid cache
invalidation, and CLI output to visit the collection.
- Use block-local context menus and existing Markdown getters so copy
commands target the clicked part rather than another part's cached
state.
- Keep existing link/mention formatting, resource-button and blob-label
search behavior, compact prompt mentions, and original user-message
chunks.
- Preserve the separator between a leading resource link and subsequent
direct or streamed text.

Release Notes:

- N/A