fix: handle malformed JSON in tool_call.function.arguments gracefully

[EkmalRey]

test body

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Response from ADK Triaging Agent

Hello @EkmalRey, thank you for creating this PR!

We noticed a few items from our contribution guidelines that need to be addressed before this PR can be reviewed:

• Contributor License Agreement (CLA): It looks like the CLA check has failed. Please make sure to sign the Google CLA. You can check the status and sign the agreement at cla.developers.google.com.
• PR Description: The current description is "test body". Please describe the bug or feature directly within the PR description, or associate an existing issue with this PR.
• Logs / Verification: Since this is a bug fix, please provide logs or command outputs showing the behavior before and after the fix is applied to help reviewers.
• Testing Plan: Please include a testing plan section in your PR description outlining how you verified these changes.

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