fix(frontend): export chat handles compacted MESSAGES_SNAPSHOT events

[derekhiggins]

When users export a completed session's chat (as Markdown, PDF, or to Google Drive), only the initial prompt is included — the rest of the conversation is missing. This makes the export feature essentially unusable for reviewing or sharing finished sessions.

The root cause is that the backend compacts finished sessions by replacing streaming events (TEXT_MESSAGE_, TOOL_CALL_) with a single MESSAGES_SNAPSHOT event. The export code only knew how to process streaming events, so all conversation content was silently dropped after compaction.

Add blocksFromSnapshot() to extract conversation blocks from MESSAGES_SNAPSHOT messages, including tool calls and results. When a snapshot is present it takes priority over streaming events since it is the canonical source after compaction.

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Done in ambient-code, Needs review (I got no knowledge of this codebase)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: b6637244-0afb-4344-9c6d-7761fe74efc5

📥 Commits

Reviewing files that changed from the base of the PR and between 7b6f4eb and 3e5b241.

📒 Files selected for processing (2)

• components/frontend/src/utils/__tests__/export-chat.test.ts
• components/frontend/src/utils/export-chat.ts

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Walkthrough

The changes add support for handling MESSAGES_SNAPSHOT events in chat export functionality. A new SnapshotMessage type and blocksFromSnapshot() helper convert snapshot data into ConversationBlock structures, with snapshot messages taking precedence over streaming events when available. Comprehensive test coverage validates message content rendering, tool call embedding, error handling, and message ordering.

Changes

Cohort / File(s)

Summary

Messages Snapshot Support components/frontend/src/utils/export-chat.ts, components/frontend/src/utils/__tests__/export-chat.test.ts

Added SnapshotMessage type and blocksFromSnapshot() helper to convert MESSAGES_SNAPSHOT events to conversation blocks, including user/assistant messages, embedded tool calls, and tool results. Updated assembleBlocks() to detect and prioritize snapshot data over streaming events. Extensive test coverage for snapshot message handling, tool call rendering, error cases, message precedence, and conversation ordering.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly describes the main change: adding support for MESSAGES_SNAPSHOT event handling in the export-chat functionality, which is the core issue addressed in this PR.
Description check	✅ Passed	The description clearly explains the problem (missing conversation content in exports), root cause (MESSAGES_SNAPSHOT compaction), and the solution (blocksFromSnapshot function), all directly related to the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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