refactor(framework): Refactor model task messages

[danielnugraha]

Issue

Description

Related issues/PRs

Proposal

Explanation

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Address LLM-reviewer comments, if applicable (e.g., GitHub Copilot)
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

[Copilot]

Pull request overview

This PR refactors typed “model task messages” by moving them into a dedicated flwr.supercore.task_message package, introducing shared utilities/validation, and updating internal imports to the new module path.

Changes:

• Introduces TaskMessage base class plus shared payload serialization helpers/constants under flwr.supercore.task_message.
• Re-implements ModelRequest/ModelResponse on top of TaskMessage with centralized validation helpers.
• Updates task-process code and tests to import model task messages from the new location and removes the old flwr.supercore.model_message module.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
framework/py/flwr/supercore/task_process/model/task.py	Updates import to new task_message.model_message location.
framework/py/flwr/supercore/task_process/agent/session.py	Updates import to new task_message.model_message location.
framework/py/flwr/supercore/task_message/validation.py	Adds reusable validation helpers for task message JSON payloads.
framework/py/flwr/supercore/task_message/utils.py	Adds shared payload <-> message content serialization helpers.
framework/py/flwr/supercore/task_message/model_message.py	Adds ModelRequest/ModelResponse implemented on top of TaskMessage.
framework/py/flwr/supercore/task_message/model_message_test.py	Updates tests to import from the new module path.
framework/py/flwr/supercore/task_message/constant.py	Adds constants for payload keys and default TTL.
framework/py/flwr/supercore/task_message/base.py	Adds TaskMessage base class and typed parsing helper.
framework/py/flwr/supercore/task_message/init.py	Introduces new package marker/docstring.
framework/py/flwr/supercore/model_message.py	Removes the previous monolithic implementation.

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