fix: remove the latest message from the user that does not have any answer yet

[kurokobo]

Checklist:

Important

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• Please open an issue before creating a PR or link to an existing issue
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Description

To resolve issue #9041, #9122 was merged, but #9122 is not functioning as expected.

#9122 is attempting to remove only the user's latest message (one that has not yet received a answer) using pop().

https://github.com/langgenius/dify/pull/9122/files#diff-32253f22ce6d4d0b26560678c8de9dc9fe3d1b11bc4f912b008152c8ed9b4380R63-R64

However, the latest message in thread_messages is at the beginning ([0]), not at the end ([-1]). Therefore, to check for an answer and pop(), it should be for the beginning ([0]), not for the end ([-1]).

Fixes #9296

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, included: Dify Document
• Improvement, including but not limited to code refactoring, performance optimization, and UI/UX improvement
• Dependency upgrade

Testing Instructions

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• The chat can continue for more than two rounds with Gemini
• By enabling the follow-up feature, suggestions related to the initial answer will be provided

Context

• The regenerate feature was introduced in feat: regenerate in Chat, agent and Chatflow app #7661
• This caused a new issue Follow-up (suggested questions) does not refer to the most recent message #9041
• The fix for the Follow-up (suggested questions) does not refer to the most recent message #9041 was made in fix: follow-up (suggested questions) does not refer to the most recen… #9122
• This caused a new issue The latest message from user is included in the history of threads, causing an error in Gemini from the second round onwards #9296

[kurokobo]

@xuzuodong
F.Y.I.

[xuzuodong]

Hi, sorry for causing this. 😥

I can reproduce this and confirm that this PR is the correct way to fix #9041.

And my understanding for the reason why only Gemini is responding error is because Gemini doesn't allow empty content on any past messages. Is that correct?