fix: render multimodal session content safely

[ruben2000de]

Agent disclosure: This change was prepared, tested, and submitted by Metis, an AI coding agent, acting on behalf of @ruben2000de. Ruben authorized the contribution and validated the fix on his Pixel 8a.

Bug Description

Opening a session that contains OpenAI-style multipart message content shows a blank/white chat screen instead of rendering the conversation.

ADB logcat reports:

type 'List<dynamic>' is not a subtype of type 'String?' in type cast
#0 _ChatScreenState._buildBody (chat_screen.dart:690)

Root Cause

ChatScreen cast every message's content field directly to String?. Hermes sessions can also contain OpenAI-compatible content arrays, for example text parts and image_url parts. The unchecked cast throws during widget construction, leaving the screen blank.

Fix

• Add messageContentToText to normalize both legacy string content and multipart content arrays.
• Preserve text parts and render image/file parts as safe placeholders.
• Render unknown structured parts as an explicit placeholder instead of crashing the whole chat view.
• Use the normalizer for user/assistant bubbles, filtering, and tool-message extraction.
• Add regression coverage for strings, text arrays, mixed text/image content, nulls, and unknown parts.

How to Verify

1. Connect the Android client to a Hermes gateway.
2. Open a session containing a message whose content is a list with text and/or image_url parts.
3. Confirm the session renders instead of showing a blank screen.

Test Plan

• Added regression tests for legacy and multipart content.
• flutter analyze --fatal-infos — no issues found.
• flutter test — all 37 tests passed.
• Manually reproduced on a Pixel 8a with the v1.0.9 APK, then verified the same session renders in the patched side-by-side build.
• Checked ADB logcat after opening the affected session; no Flutter exception remains.

Maintainer Notes

• Review scope is intentionally small: one commit, three files.
• No dependency, configuration, storage, API, or migration changes.
• Maintainer edits are enabled on the source branch.
• GitHub Actions is awaiting the standard first-time fork-contributor approval; the same analyze/test commands have already passed locally.

Risk Assessment

Low — the change is isolated to display-time conversion of message content. Existing string messages remain unchanged, while non-string content now degrades to text/placeholders instead of throwing during rendering.

[rusty4444]

Reviewed by Hermes Agent.

Approve: the new messageContentToText helper correctly preserves legacy string content, handles multimodal text/image/file parts without throwing, and replaces unsafe string casts in chat rendering. Targeted local check passed: flutter test test/message_content_test.dart.

Note: this PR has no GitHub CI checks reported, so automation will not merge it despite approval.