feat(chat-app): refresh Qwen3.5 presets and harden multimodal text rendering

[leehack]

Summary

• switch the chat app's built-in Qwen catalog to Qwen3.5 (0.8B/2B/4B/9B), add mmproj wiring, and remove older Qwen2.5/Qwen3 defaults from the in-app model list
• tune safer multimodal defaults for Qwen3.5 (contextSize: 8192, maxTokens: 1024) to reduce context-slot/decode failures on image prompts
• fix Flutter paint crashes from malformed UTF-16 during streaming by aligning incremental reveal boundaries and sanitizing message/tool text render paths
• update chat app docs and changelog to match the new model lineup, architecture layout, and troubleshooting guidance

Testing

• flutter analyze (example/chat_app)
• flutter test (example/chat_app)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.81%. Comparing base (6ebbe62) to head (08481fd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
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  Coverage   76.81%   76.81%           
=======================================
  Files          66       66           
  Lines        8579     8579           
=======================================
  Hits         6590     6590           
  Misses       1989     1989           

Flag	Coverage Δ
unittests	76.81% <ø> (ø)

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