feat(tui): show user pasted content

[joshbarrington]

Summary

This PR enhances the TUI to render pasted content back to the user - automatically collapsing large pasted file contents (exceeding 30 lines) into a toggleable view, improving overall chat readability and scrolling performance.

Key Changes

• Show User Pasted Content: Currently pasting multiple lines into docker agent TUI renders as @paste-1 for example. This shows the pasted content back to the user to allow for better readability.
• Collapsible User Messages: Added expand/collapse functionality to user messages exceeding 30 lines (rendered via messages.go and message.go).
• Interactive Toggles: Implemented a generic toggleableView interface in messages.go to handle click-to-toggle interactions on message views (now supporting both reasoning blocks and long user messages).
• Robust File Replacements: Modified editor.go to sort paste attachments by length descending before inline string replacement (@paste-1 vs @paste-10), preventing partial match corruption, while fully retaining proper order of standard metadata file attachments.
• Performance Enhancements: Prevented memory allocation spikes by utilizing zero-allocation string ops (strings.Count and strings.SplitN) instead of heavy array allocations (strings.Split) during truncation calculations.
• Coverage: Added comprehensive TestUserMessageCollapsible and TestUserMessageNotCollapsible unit tests to secure boundary math and UI layout mechanics.

[rumpl]

@joshbarrington thanks! Could you show me some screenshots? I won't have the time to check out your branch

[joshbarrington]

@rumpl

Usual user pasted content

Long user pasted content (> 30 lines) with expand option

Expanded content with collapsible option

[joshbarrington]

Current behaviour on main:

[dgageot]

/review

[docker-agent]

Assessment: 🟡 NEEDS ATTENTION

This PR renders pasted content in the TUI with collapsible messages for large pastes. The implementation is solid overall — the sort-by-length trick for placeholder disambiguation is clever, the cache invalidation on toggle is correct, and the toggleableView interface dispatch is sound.

One medium-severity issue was found in the paste expansion loop.

[docker-agent]

[MEDIUM] Paste content expansion can corrupt messages if pasted content contains @paste-N strings

The loop in resetAndSend replaces each @paste-N placeholder with the paste's content using strings.ReplaceAll. Sorting by name-length descending correctly prevents @paste-1 from partially matching @paste-10 in the original input — but it doesn't guard against a later paste's ReplaceAll matching an @paste-N string that was introduced by a previous paste's content.

Trigger scenario:

1. User pastes two items: paste-1 and paste-2
2. The content of paste-1 happens to contain the literal string @paste-2 (e.g., a shell script, documentation, or any text referencing another paste)
3. After paste-1 is expanded (longer name goes first), content now contains @paste-2 embedded in the replacement text
4. When the loop reaches paste-2, strings.ReplaceAll finds that injected @paste-2 and replaces it with paste-2's content — double-expanding and corrupting the message

Suggested fix: Replace only the first occurrence in the original content position, or process on a copy of the original content string (tracking expansions separately), or escape @paste-N references in the source string before looping:

// Option: replace on a snapshot so injected tokens aren't re-processed
original := content
for _, att := range pastes {
    content = strings.ReplaceAll(original, "@"+att.Name, att.Content)
    original = content // or accumulate replacements on original only
}

A cleaner approach is to do all replacements against the original content value and accumulate them without re-scanning previously expanded text.