feat(lite_llm): add text/plain file support

[134130]

Description

• fixes: litellm not support upload txt file #1940

Testing plan

• Upload the file content with mimeType 'plain/text'
• Note that source code files like shell script are not able to processing.

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• similar PR: feat(lite_llm): add PDF support #1855

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[134130]

[hangfei]

Response from ADK Triaging Agent

Hello @134130, thank you for creating this PR!

It looks like the CLA check is failing. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA). You can visit https://cla.developers.google.com/ to see your current agreements or to sign a new one.

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