Rename single_node_default to single_node_exchange

[Copilot]

single_node_default is ambiguous—it sits next to single_node_reductions in definitions.py but doesn't convey that it's an exchange runtime. Renamed to single_node_exchange to match its type (SingleNodeExchange) and mirror the naming of its sibling.

• Renamed the module-level variable in decomposition/definitions.py
• Updated all 11 consuming files across model/common, model/atmosphere/diffusion, and model/atmosphere/advection (source + tests)

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[jenkins-apn]

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[jenkins-apn]

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[jcanton]

cscs-ci run default

[github-actions]

Mandatory Tests

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• cscs-ci run default
• cscs-ci run distributed

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[jcanton]

cscs-ci run distributed