fix(wechat): Service API text requests fail on streaming-only models, and image fallback should upload pic_url as local_file

[AlexMultiAgent]

Summary

When using bikeread/dify_wechat_plugin with Dify Service API and a streaming-only model such as qwen3.5-flash, the plugin may return AI did not give a reply for follow-up text messages.

Image handling also has a fallback gap: if WeChat media_id download fails because the server IP is not in the WeChat whitelist, the plugin should still process the image reliably by downloading pic_url itself and uploading it to Dify as local_file.

There is also a conversation recovery issue: if a cached conversation_id becomes stale, Dify returns Conversation Not Exists, and the plugin should clear the cached conversation and retry automatically.

Reproduction

1. Install and configure the WeChat plugin with a Dify chat app.
2. Configure the plugin to call Dify through Service API.
3. Use a streaming-only model such as qwen3.5-flash.
4. Send an image to the public account.
5. Send a follow-up text such as 这是什么.

Actual Behavior

• The image request may succeed, but the follow-up text can return AI did not give a reply.
• If a cached conversation is stale, Dify returns Conversation Not Exists.
• If WeChat media API access is blocked by IP whitelist, the plugin cannot fetch media_id directly.

Relevant errors observed locally:

This model only supports incremental_output set to True.
Conversation Not Exists
invalid ip xxx.xxx.xxx.xxx, not in whitelist

Expected Behavior

• Text requests should work with streaming-only models.
• If media_id fetch fails, image requests should still work by downloading pic_url and uploading it to Dify as local_file.
• If a cached conversation becomes stale, the plugin should clear it and retry once automatically.

Root Cause

• Service API text requests use response_mode=blocking, which breaks on models that require incremental output.
• When media_id fetch fails, the current fallback is fragile.
• Cached stale conversation_id is not always auto-recovered.

Suggested Fix

• Always use response_mode=streaming for Service API chat requests.
• Parse SSE responses for both text and image requests.
• If Dify returns Conversation Not Exists, clear cached conversation and retry once.
• If WeChat media_id fetch fails, download pic_url inside the plugin and upload it to Dify /files/upload, then send it as local_file.
• Keep remote_url only as the last fallback.

Notes

I already verified this locally with qwen3.5-flash and Dify Service API. If needed, I can open a PR with the exact patch.

[AlexMultiAgent]

Opened PR #38 with a local fix and validation notes: #38