Replace repository plan API with subscription plan endpoint

[jfrench9]

Summary

Refactors the subscription management API by replacing the repository-scoped change_repository_plan endpoint with a broader change_subscription_plan endpoint, and enhances the graph subscription response model with additional fields.

Key Accomplishments

• Renamed and refactored API endpoint: Migrated change_repository_plan.py to change_subscription_plan.py, shifting from a repository-centric plan change to a subscription-level plan change. The new implementation includes expanded logic and improved handling (~80 lines added, ~36 removed).
• Enhanced graph subscription response model: Added new fields to GraphSubscriptionResponse to capture richer subscription metadata, improving the data available to consumers of the subscription API.
• Updated get_graph_subscription endpoint: Modified the graph subscription retrieval logic to align with the new response model structure and ensure consistency across the subscription API surface.

Breaking Changes

⚠️ Yes — this is a breaking change.

• The change_repository_plan API endpoint has been removed and replaced with change_subscription_plan. Any clients or integrations referencing the old endpoint or its associated models will need to be updated.
• The GraphSubscriptionResponse model has new fields, which may affect deserialization or schema validation in downstream consumers depending on how they handle unknown/additional fields.

Testing Notes

• Verify that the new change_subscription_plan endpoint correctly handles plan change requests that were previously routed through change_repository_plan.
• Validate that the get_graph_subscription endpoint returns the updated response model with all new fields populated correctly.
• Ensure that no references to the removed change_repository_plan endpoint remain in client code or integration layers.
• Test edge cases around subscription plan transitions and confirm the enriched response model is serialized/deserialized properly.

Infrastructure Considerations

• Downstream services and API clients consuming the subscription endpoints will need coordinated updates to reference the new endpoint path and handle the updated response schema.
• API documentation and SDK bindings should be regenerated to reflect the renamed endpoint and expanded model definitions.

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Branch Info:

• Source: feature/graph-subscription-change
• Target: main
• Type: feature

Co-Authored-By: Claude noreply@anthropic.com