[Bug]: Pasting long text into chat produces no output and no error with CherryIN Google Gemini 3.1 Pro (empty assistant turn)

Issue Checklist

• I understand that issues are for feedback and problem solving, not for complaining in the comment section, and will provide as much information as possible to help solve the problem.
• My issue is not listed in the FAQ (https://github.com/CherryHQ/cherry-studio/issues/3881).
• I've looked at pinned issues and searched for existing Open Issues, Closed Issues, and Discussions, no similar issue or discussion was found.
• I've filled in short, clear headings so that developers can quickly identify a rough idea of what to expect when flipping through the list of issues.
• I've confirmed that I am using the latest version of Cherry Studio.

Platform

macOS

Version

v2.0.0-rc.5

Bug Description

In a normal chat using model cherryin::google/gemini-3.1-pro-preview, pasting a long block of text (Cmd+V) into the composer and sending produces NO assistant output and NO error — the turn appears empty. Saving the exact same content as a .txt file and attaching it sends fine and gets a normal reply.

Steps To Reproduce

1. Chat model: CherryIN → google/gemini-3.1-pro-preview.
2. Copy a long text (~5k+ chars, i.e. >1500 chars) and paste directly into the composer (Cmd+V), then send.
3. The assistant turn produces nothing (only an empty bubble / step-start), no error message.
4. The same content saved as a .txt file and attached (or dropped) sends fine and gets a full reply.

Expected Behavior

Long pasted text should either be sent as inline text and get a reply, or be converted to an attachment (as designed for >1500 chars) and replied to. It should never silently produce an empty turn; if the upstream short-circuits, Cherry should surface an error.

Additional Context

Evidence from local DB (message table, timestamps 06:49–06:52, same model):

• Failing turn (topic bc8c39bf): user message is a 17,959-byte plain-text message (no attachment). Assistant message(s): parts [{"type":"step-start"}] only; stats inputTokens=0, outputTokens=0, generationDurationMs≈1500.
• Working turns (same model; 52KB content attached as .txt, topics 91ccad14 / 8a49371e): assistant inputTokens=20256/32468, outputTokens=6559/6341, model reads the file via the read_file tool and replies fully.
  Code path: the composer editor's handlePaste (HorizontalScrollContainer-BO7Df1cE.js) intends to convert pasted text >1500 chars into a temp .txt attachment (shouldDelegateLongTextPasteToFileHandler → handlePaste writes temp_file_*_pasted_text.txt), but in this case the text went through as inline text. CherryIN routes google/* to the Gemini endpoint (https://open.cherryin.net/v1beta/models).
  The empty reply (0 input / 0 output tokens, ~1.5s) looks like the CherryIN gateway returns an empty successful response for a very long single text user message — worth reporting to CherryIN too. Related: #16329 removed the long-paste-to-file settings while keeping the auto behavior; the auto-conversion path should be verified.