[Contracts] Add Notification Template Registry

[Abd-Standard]

Description

Introduce an on-chain registry for reusable notification templates. This allows senders to reference predefined templates instead of repeatedly storing identical notification structures.

Tasks
Create template registration functions.
Allow template updates where appropriate.
Emit template registration events.
Validate template existence before use.
Add comprehensive unit tests.

Acceptance Criteria
Templates can be registered successfully.
Invalid template references are rejected.
Events are emitted for template actions.
Tests cover registration and retrieval.

Contributors start work by running:
git checkout -b contracts/template-registry

[Emmanuellsensai]

hi maintainer, i would like to work on this issue

[AbuJulaybeeb]

Hi Maintainer, haven reviewed the description of the issue, and well experienced with Js Library,
I would like to hop on the issue and sure to deliver at ETA 2hrs

[mrnetwork0001]

I am highly interested in implementing the notification template registry to optimize state usage and prevent redundant data storage.

My background includes writing clean, tested smart contracts, handling template updates securely, and validating references on-chain.

I can begin right away and deliver a thoroughly verified, gas-efficient registry module.

[clintjeff2]

Hello, I have 6 years of experience working on tasks like this and handling similar challenges across backend, frontend, and smart-contract development. I have carefully read and fully understood the task requirements, and I am confident I can deliver a clean, efficient, and well-tested solution promptly. I always ensure high-quality implementation, maintainability, and adherence to best practices.

Pull request in 2hours

[GBOYEE]

For the notification template registry (#352), I'll add a NotificationTemplateRegistry contract module with:

1. create_template(id, name, template_data) — stores reusable notification structure
2. get_template(id) — retrieves template for reference
3. use_template(notification_id, template_id) — links a notification to a template

Storage pattern follows existing Soroban conventions with DataKey::Templates map. Will include tests for template CRUD and template-linked notification creation. Ready on assignment.