CROSSLINK-305 Add default template endpoint

[JanisSaldabols]

No description provided.

[Copilot]

Pull request overview

Adds a new “template defaults” concept alongside existing state-model/batch-action defaults, exposing it via the patron-requests API so clients can retrieve built-in template presets (e.g., for scheduled pullslip summary emails).

Changes:

• Added templateDefaults (including an initial HTML email template) to misc/state-models.yaml.
• Extended the embedded state model config parsing and added GetStateModelTemplateDefaults() to return template defaults defensively.
• Introduced a new GET endpoint /state_model/templates in OpenAPI and implemented the corresponding API handler.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
misc/state-models.yaml	Adds a built-in default template definition (scheduled pullslips email).
broker/patron_request/service/statemodel.go	Adds TemplateDefaults to embedded config and exposes GetStateModelTemplateDefaults().
broker/patron_request/api/api-handler.go	Implements GetStateModelTemplates handler to serve the defaults.
broker/oapi/open-api.yaml	Defines the new /state_model/templates endpoint returning CreateTemplate[].