Allow lookup of subscriptions by Internal Name

🔍 Problem Statement
As an admin, I experience being unable to search for price plans by Internal Name during subscription setup, which slows onboarding and increases errors when many plans share similar product names.

💡 User Story
As an admin, I want to type an Internal Name in the plan search field when creating a subscription, so that I can find the correct plan quickly without scrolling through a long list.

🎯 Definition of Done
- Given the subscription creation flow, when typing in the plan search field, then results include matches against both product name and Internal Name.
- This change impacts the UI (plan search/select component) and API (plan search endpoint — add internalName filter).
- Limitations: Internal Name search is case-insensitive; partial match is accepted.

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We would like to be able to look up price plans when setting up new subscriptions using the Internal Name field. For example: if we have a product called “The Science of Teaching”, and the Internal Name is SCIENCE123, when we go to enter a new subscription, we would like to be able to look up price plans based on the Internal Name (SCIENCE123) instead of having to type out the Product Name (The Science of Teaching). Thank you for your consideration.