feature(subscription): scope wait callbacks at runtime

[Galfurian]

What this PR does

Adds support for scoping subscription callbacks to the active execution context, so wait conditions and runtime subscriptions use the correct listener instead of sharing callback state across executions.

Why

Previously, subscription callbacks were effectively global to the variable node, which made nested or concurrent executions brittle. The missing piece was keeping the callback tied to the active context that created the subscription.

Main changes

• Added context-scoped callback storage on ExecutionContext.
• Threaded the active callback through wait-condition subscription setup.
• Updated composite method execution to carry the scoped callback into child subscriptions.
• Added a shared SubscriptionCallback type alias to avoid repeating the callback signature.
• Added/updated tests to cover scoped wait-subscription behavior.
• Added a small subscription example showing multiple listeners and clear notification flow.

Validation

• Focused behavior/composite test suites pass after the change.
• The subscription example runs and shows the expected callback sequence clearly.