DateTime stamp format issues when using mixed A2A language implementations (python + Java)

### What happened?

When using a python based Agent which is fronted with a Java based orchestrator, there is a conflict with datetime format pattern.
Issues:
When sending a message from the Java orchestrator to the python agent and getting response, the unmarshall of the response throws an error about the datetime format. Java code expects a "yyyy-MM-dd'T'HH:mm:ss.SSSSSS" format while the python back end sent date time in "yyyy-MM-dd'T'HH:mm:ss.SSSSSSSSSXX" format.
Workaround:
Changed the python A2A library to send it in the Java expected format.

### Relevant log output

```shell
Failed to send message: com.fasterxml.jackson.databind.exc.InvalidFormatException: Cannot deserialize value of type `java.time.LocalDateTime` from String "2025-07-02T18:43:53.974294+00:00"
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look at the A2A protocol message serialization/deserialization code in both the Java orchestrator and Python agent libraries. Identify where datetime formatting is configured. The fix likely involves aligning the timestamp format in one library to match the other's expectation, possibly by modifying a date formatter pattern. Test by sending a message between the two and verifying the datetime field parses correctly.

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