422 Unprocessable Entity in versions 2.6.0, 2.7.0, 2.8.0 when trying to save an eval case

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**Describe the Bug:**
https://github.com/google/adk-python/issues/6336 seems to still persist in versions 2.6.0, 2.7.0, 2.8.0.
Upon trying to edit an eval case on an existing eval set, and hitting save (on the top right) this error is seen:
`HTTP/1.1" 422 Unprocessable Entity`

**Steps to Reproduce:**
Similar to in https://github.com/google/adk-python/issues/6336 but adding steps anyway:
- adk web
- Go to localhost:8000
- Create a new session
- Once agent responds, got to evals tab and create a new eval (+ new)
- Open eval set, and hit +current session (adds the session to eval set)
- Once added, session shows in the evalset, hit edit and try to edit some message in the session
- Once edit done, hit save on the top right
- Save does not happen. I see this on the logs: `HTTP/1.1" 422 Unprocessable Entity`

**Expected Behavior:**
The eval set should update the eval case and give 200 OK

**Observed Behavior:**
`HTTP/1.1" 422 Unprocessable Entity`

**Environment Details:**

- ADK Library Version (pip show google-adk): 2.6.0, 2.7.0, 2.8.0
- Desktop OS:** macOS
- Python Version (python -V): 3.11.6

**Model Information:**

- Are you using LiteLLM: No
- Which model is being used: gemini-2.5-pro

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**Regression:**
Did this work in a previous version of ADK? If so, which one?
no