[rollout] fix: remove redundant system message added in ToolAgentLoop._handle_processing_tools_state

[m-Just]

What does this PR do?

In ToolAgentLoop._handle_processing_tools_state, apply_chat_template adds a default system message to tool response messages but the system message is redundant and should be removed. This PR removes the redundant system message.

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Test

Tested with my own experiment, and it worked as expected.

API and Usage Example

No API change.

Design & Code Changes

Simply removed the system message.

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[gemini-code-assist]

Code Review

This pull request aims to fix an issue where a redundant system message was being added when processing tool responses. The change correctly identifies the need to strip this system prompt. However, the implementation of stripping it by slicing token IDs is not fully robust due to the nature of some tokenizers, and could lead to subtle bugs. I've provided a critical review comment with a suggested code change for a more reliable implementation that operates on the string level before tokenization.