fix(framework): Make RUNNING transition atomic across replicas

[psfoley]

Issue

Description

Related issues/PRs

Proposal

Explanation

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

[Copilot]

Pull request overview

This PR aims to make the STARTING -> RUNNING run-status transition safe under multi-replica contention in the SQL-based LinkState implementation, and adds concurrent tests to validate uniqueness/claim behavior across SQLite-backed replicas.

Changes:

• Add an atomic UPDATE ... WHERE ... RETURNING claim path for Status.RUNNING transitions in SqlLinkState.update_run_status.
• Improve/standardize an error log message related to invalid sub_status values.
• Add multi-threaded, multi-replica SQLite tests to ensure unique claiming of messages and the RUNNING transition.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
framework/py/flwr/server/superlink/linkstate/sql_linkstate.py	Introduces an atomic update path for claiming STARTING -> RUNNING across replicas and adjusts an error message.
framework/py/flwr/server/superlink/linkstate/linkstate_test.py	Adds file-based SQLite concurrency tests to validate unique claiming under contention and atomic RUNNING transitions.

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